By Gregor Paul in Tokyo
Wallabies first-five Bernard Foley will never forget the mistake he made in Brisbane 2017 when he changed his running angle, believing that by doing so he would cruise past Ofa Tuungafasi unscathed.
The 130kg All Blacks prop, with the speed and agility of a dancer half his size, shifted his feet, adjusted his body position and exploded into Foley's rib cage with such force that some in the crowd genuinely feared the Australian wouldn't be able to get up.
Tuungafasi's back was parallel to the ground, his knees were bent, his first contact point was his shoulder, his head safely out the way and then the arms locked round Foley in what was not only one of the great tackles for the ages, but close to being technically perfect.
And it wasn't a lucky hit by Tuungafasi. It was a tackle executed not by chance but by design as the All Blacks are meticulous and relentless behind the closed doors of their training grounds when it comes to getting their tackling technique right.
They are conditioned to not just play for 80 minutes, but to play at a lower body height than anyone else.
There's a basic philosophy driving this which is that to dominate a game of collisions, you have to enter contact at a lower height than the opposition.
To tackle effectively, the tackler has to be lower than the ball carrier and so the thinking is reversed when the All Blacks practice their ball carrying, which is why conditioning coach Nic Gill has designed endless drills to enable the players to execute their core skills, particularly tackling, at safe and effective heights.
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Hours have been spent bear crawling – big men trying to propel themselves with their hands on the ground, their faces just inches from the turf.
Similar amounts of time have been spent getting the players used to being flat on the ground and back to their feet and into their running without their bodies becoming upright.
The value of those drills can be seen when the All Blacks are defending their line, almost on their stomachs so they can get underneath the ball carrier and either repel him or hold him up.
And then there is the actual tackling itself – it's all about staying low, retaining a dynamic body position and keeping the head down, eyes up to focus on hitting below the ball.
As All Blacks hooker Codie Taylor, says: "We drive really hard about tackling under the ball. If you are under the ball usually you are in a good place where you are not going to be penalised."
But it's the inclusion of the word "usually" which has been drawn into focus at this World Cup where five red cards and 17 yellow have been shown, two of which have been to All Blacks Tuungafasi and Nepo Laulala.
Some have been inexcusable – red cards in any day and age. But others have been the victims of World Rugby's non-negotiable desire to eliminate all contact to the head.
The global body conducted research after the last World Cup which found that 76 per cent of concussions occur in the tackle, with 72 per cent of those to the tackler, and that head injury risk is 4.2 times greater when tacklers are upright.
The statistics told them everything they needed to know about where the danger sits in modern rugby and they have been on a crusade since November 2016 to penalise those who tackle high.
The pressure has increased year by year on referees to be more punitive with World Rugby justifying their stance on the body of evidence they have gathered.
But the research was conducted in professional club games, which are not played at the same speed or intensity as tests nor do they feature such explosive athletes.
The intent to rid the game of needless head injuries as a consequence of poor tackling technique is globally supported, but this World Cup has brought into view the absurdity of trying to eliminate all head contact in a collision sport.
Often accused of being vague and indecisive, World Rugby produced clear and inflexible guidelines this year that say any tackle contact to the head will be red-carded with the referee able to judge whether any mitigating circumstances could be factored in to reduce the sanction to yellow.
World Rugby, no matter the consequences to their show-piece event, are entrenched in their position now, having publicly criticised referees for not applying the law appropriately in the early games.
It has meant that every game at this tournament has come with a big-screen drama where a collision involving the head, deliberate or accidental, has been referred to the TMO, collective breath held by the accused player and his team-mates as they await their fate.
"Once the review signal comes out it is pretty clear if it looked half dangerous in real-time then someone is going to go to the bin," says Taylor.
"Whether it is a yellow or a red ... the only thing you can hope is that it is not going to be red because that really does affect the game."
The inflexibility of the laws is in danger of turning the tournament into farce – innocent and guilty players being punished alike when it is clear that not all head contacts are the same.
It makes no sense to treat them as such and every coach at the World Cup says it lacks context and empathy for the reality of a collision sport.
Tuungafasi's yellow card against Namibia saw him penalised for executing a tackle at a similar body height to the one that snapped Foley.
His head was in the right place, his eyes were up, he was on his toes with his knees bent.
In one instance he's a hero, and in the other he's a villain and it is all because of one thing – the body position and height of the ball carrier.
Foley had been upright when Tuungafasi hit him, but Namibia's Darryl de la Harpe was falling when the All Blacks prop made contact.
It was the same for Laulala, whose body height was barely a metre from the ground when he tackled Lesley Kim.
In both cases the head contact was neither malicious and nor was it the consequence of poor technique, although Laulala's arm was late to come in line with his shoulder which made it look worse than it was.
And this is the impossible new dimension the All Blacks, and every other team at the World Cup faces, that they can be carded because a ball carrier falls into them, leaving them without enough time to adjust.
The ball carrier whose height is dropping as they come into contact is now the most deadly weapon in the game as it carries a high risk of the tackler being penalised.
"It is a rock and a hard place sort of thing," says All Blacks prop Angus Ta'avao. "You don't want for there to be contact with the head and we are not going out there to intentionally hurt people.
"The way the boys tackled against Namibia it is quite tough ... it's a low tackle and knowing Ofa and Nepo the way they tackle it is usually quite low as it is, but you have to understand that there can't be any contact with the head so you have to be aware.
"We always talk about having our arms in tight and not having them swing around to take away that risk but we have seen in this competition what can happen ... guys having their World Cup pretty much finished because of a red card and a ban."
It has reached the extent that some coaches have privately confessed that it may become a deliberate tactic for teams to drop their body heights late as they come into contact as a means to try to invoke cards in the knock out rounds.
But World Rugby are not going to back down and for all the work the All Blacks have done to lower their body positions and refine their technique for this tournament, they are going to have to do more.
"That seems to be the nature of the World Cup at the moment but I don't have the answer," says Coles.
"We have just got to find a way to avoid it. When guys are falling you almost have to give up the tackle so you don't get caught hitting him high. So we have to do some work and get better at that because yellow cards can be very costly.
"It is hard but we are going to have to find a way."
The All Blacks coaching staff aren't so sure they have the answer either, but this week the team will spend extra time on trying to find solutions to this specific problem.
An already heavy emphasis on defensive technique is going to be heavier as the All Blacks are not prepared to give up their champion status to a red card.
The key, as they see it, is for tacklers to keep their elbows tight to their body and their hands in front, visible like they are preparing to catch the ball.
That allows tacklers to adjust late if needs be and potentially look to get their arms underneath the ball carrier or throw them over the top so as they make contact well down the back.
Most importantly, though, it avoids creating an incriminating picture where the tackler's arm trails the shoulder and looks like it has been swung deliberately into the ball carrier's head.
"Are we dealing with a ball carrier falling over in the middle of a tackle?," says coach Steve Hansen.
"Because most of the other tackle technique stuff we have worked on for years and we are okay. We are working on that. The game is incredibly difficult under the current guidelines when players are falling at your feet.
"There are certain things we are going to have to make sure we do. You can't have an arm behind the shoulder because it looks like a swinging arm.
"It is about being able to recognise quickly that they (the ball carrier) are falling and about either pulling out of the tackle or trying to do it differently.
"By and large most people are clean players. But our game is very fluid and in a second, something you thought you saw can change, particularly for the big boys. It's like a big boat versus a little boat isn't it? The Titanic didn't move quick enough and sunk."
No one can be sure why World Rugby have decided to be so militant at this tournament. A number of the world's best coaches - including Hansen - lobbied collectively before the World Cup to soften the stance and plead for some kind of common sense to be applied.
But to no avail as World Rugby say their stance is driven exclusively by player welfare and yet after Owen Farrell was hit high by Tomas Lavinni in a collision that saw the Argentinian lock sent off, the Englishman wasn't taken for a Head Injury Assessment.
It's inconsistencies such as that which have failed to make the players feel any safer since the stricter regulations were applied.
"Whether or not you feel safer I am not sure because it is a contact sport and those things are going to happen," says Taylor.
Speaking to the Herald last year All Blacks flanker Sam Cane said the much bigger problem in relation to safety is being rolled out of rucks by the neck.
That, he said, sparks genuine fear into players as they are often put into the most vulnerable positions that could end in genuine tragedy.
Conversely he said it was rare indeed to encounter any malice or intent connected with a high tackle and that players accepted the inherent risks of playing a collision sport and the probability of there being accidental collateral damage.
So the current situation is being portrayed by World Rugby as a battle to force players into adopting lower body positions at the tackle. But the players see it now as a battle for rugby's soul as the very essence of the sport is at risk if they can't express themselves physically without fear of unfair recrimination.
"It's all part of the problem," says Hansen. "Our game is about intimidation and some people might not want to hear that, but that's a fact. It's about me dominating you and you do that through intimidating legally not illegally but it is a physical game, always has been, always will be and that is one of the components players, coaches and fans love about it."