There has been, in the last two Rugby World Cup cycles, at least one, if not two player signings that have had a defining influence.
In 2008, the retention of both Richie McCaw and Daniel Carter persuaded a host of senior All Blacks also to hang around for a crack at the 2011 cup.
Again, in 2011, when McCaw and Carter re-signed through to 2015, it led to the likes of Kieran Read and Sam Whitelock also making the decision to bed in for the long haul.
This time, it could be Ben Smith who is the game changer leading into the 2019 cup.
He signed last week through to 2020 and he put pen to paper despite there being a significant offer in front of him from the Pau club in France.
It was a powerful statement - that he values the challenge of winning more Super Rugby titles with the Highlanders and going to another Rugby World Cup with the All Blacks as more rewarding.
Implied in his decision is his confidence that the All Blacks in particular are a side worth hanging about to be part of and that he believes they are a reasonable chance of winning three Rugby World Cups in a row.
This is what happened in 2008. Carter and McCaw both gave clear messages to their peers that they felt the All Blacks could put the failure of 2007 behind them and make things right in 2011.
Once they committed, it made it easy for the likes of Mils Muliaina, Ali Williams and Rodney So'oialo to follow suit. Sometimes players who are wavering about their future gain clarity when a trusted and valued peer makes a decision to stay.
And Smith is of value to the All Blacks. Significant value. As widely respected as he is by a growing army of fans, his team-mates value him more.
They know, better than anyone, what he brings to the test arena. He is a calm head and a strong voice. Smith rarely makes mistakes. He is hardly ever caught behind the gainline or guilty of running up dead ends. His kicking game is improved, he pops up well as a decision-maker in the frontlines and he sees what others don't.
Most importantly, he makes the impossible possible. How many times in the past few years has it been Smith who has sparked the All Blacks' attack into life?
How many times has he shifted the momentum of a test with an outrageous play? The chase-back tackle-turnover on Manu Tuilagi in Dunedin 2014? The high ball he caught in the Rugby World Cup quarter-final when he landed, literally, on French No8 Louis Picamoles?
He's inspirational and now he's staying, there is a lift in optimism Israel Dagg may reach the same conclusion.
The All Blacks are keen for Dagg to stay until 2019 but he hasn't given any indication about his intentions. Dagg is wanted in Ireland and by the big-spending Toulon and, while he's tempted, the prospect of being part of an All Blacks back three that will definitely include Smith could be the factor that tips him towards staying.
Maybe, maybe not. But if nothing else, it will at least be harder for Dagg to walk away from the All Blacks when so many good players are choosing to stay.
It's the long-term presence of Smith that will interest him. The two were beginning to form an effortless and lethal combination in 2016 with the unexpected scenario of Dagg on the wing.
They linked cleverly, read each other well and the fact they are both so comfortable at either fullback or wing allowed them to quickly turn defence into attack.
Rugby, all said and done, is still best when it is played with mates and Smith is exactly the sort of bloke who engenders something binding and meaningful in a team environment.
Dagg will do his own thing in his own time but his decision may well be heavily influenced by what he has seen Smith do.