Everyone knows what a knock on is, don't they?
After all, players and spectators are only too willing to help the referee rule on these minor offences during a game, so it shouldn't be a problem.
Yet I know from over 50 years of officiating that pundits probably only have about a 50 percent accuracy rate.
Mind you, referees may not be much better, as I shall explain later.
A knock on occurs when a player loses possession of the ball and it goes forward, or when a player hits the ball with the hand or arm and it goes forward and touches either the ground or another player before the original can catch it again.
The obvious knock-on's, where the ball clearly goes forward after being hit by arms or usually hands, are not a problem.
It is not a knock on when a player unintentionally knocks the ball forward but recovers it before it touches anyone or anything, so play continues.
Should the ball hit another player or the ground before the original carrier regathers it, then the fumble has occurred.
Usually, a referee will look to call advantage to the opposing team in this situation so play can continue, provided the non-offenders get a clear and obvious advantage with either territory or retaining possession.
Once the ball has passed through at least two pairs of hands a referee will likely call "advantage over".
However, if the ball goes straight to the ground, not forwards, then it should not be ruled as a knock on.
This often happens when a player is trying to catch a kick low to the ground and accidentally drops it straight down, without it ever going forward.
Or, the ball goes straight through the arms and to the ground, likewise not going forward.
Often, after either of those two scenarios, the ball does travel forwards from the ground, making it even more obvious to others that it was "knocked on".
I believe that in both these situations, the ball has not been knocked on because it did not travel forward from the catcher's hands or arm, so play should continue.
But, generally speaking, referees are often quick to rule it as a knock on, taking no chances of making a wrong call in the eyes of players and spectators.
A knock on must not be intentional.
This word didn't used to be in the definition of a knock on, but was added a few years ago and has actually created a few issues for referees – just how do you decide if a knock on is intentional?
Generally speaking, intentional knock-on's occur when a player tries to intercept a pass from opponents.
The critical piece of evidence is the hand action of the player trying to intercept or catch the ball.
If the ball travels straight to the ground from the interceptor's hand, it is considered a deliberate knock on and sanctions will be applied to the player.
This could be at least a penalty, but could also be a yellow card and even a penalty try as well – if the action prevented what would have been a probable try.
But, if the interceptor knocks the ball forwards or upwards and has a reasonable chance of regathering the ball, then it would be ruled as just a knock on if he didn't manage to catch it.
Players these days tend to tackle higher on the body in an attempt to stop the ball carrier from off-loading to a team mate.
If this tackler makes contact with the ball and it travels forwards, then this is ruled as a knock on by the defender, as the ball has gone backwards from the carrier but forward from the tackler's hand or arms.
It is not a knock on if the ball travels forwards from a player's hands or arms in the act of charging down a kick by an opponent, as long as it is immediately from the kick.
There is also no knock on if the ball is ripped from a player's possession by an opponent and travels forward from the ball carrier.
There is a clear distinction here between ripping the ball forward from a player's possession, and the ball being knocked forward by a defender in the act of tackling.
As expected, there has been plenty of talk already about the performances of referees at the World Cup in Japan.
For me, the stand out so far has been Wayne Barnes, who has been right on the money most of the time.
Just watch him move into the in-goal when a try looks like being scored and you will see why he is the front runner for refereeing the final.
Provided England aren't in it. Perish the thought.