IT USED to be known as the ''spare man'', or ''loose man in defence''. Nowadays it has become the ''plus-one'' system.
It sounds complicated, but really it's not. It's not even especially new, although it has become very common in modern AFL football.
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From the old South Melbourne grounds for the Swans, Rohan Connolly lays down his tips for round six.
The public don't always understand why it happens; in fact they are infuriated when the opposition uses it against their team. Generally, it is a result of a philosophical decision by their team.
For instance, a fortnight ago, Harry O'Brien played a whale of a game for Collingwood against Port Adelaide. O'Brien had been beaten by small forwards in a couple of games; Nathan Buckley astutely observed that he needed to regain confidence.
Port was heavily criticised afterwards because O'Brien appeared to run without an opponent. But it was not that Port did not respect him, or chose not to give him an opponent.
It was philosophical. As coach Matthew Primus told On the Couch this week, the Power wanted an extra man up in the heart of the action, and that man came from their forward line.
''In the evolution of how we want to teach our players to play, we need some numbers up at the stoppages,'' said Primus. ''At the moment we can't quite go one-on-one in our forward line.''
Once Port made that decision, all Collingwood needed to do was shuffle its backs until it worked out who to leave spare. O'Brien had a smorgasbord, made more obvious by the fact Port persisted in kicking it to him. Bad decision-making and execution is a problem that no amount of strategic planning can overcome.
What Primus weighed up was the benefit he got from an extra body in the midfield against the possibility that Collingwood's spare man caused too much damage. Clubs ponder this every week and, as it happened, Essendon was watching closely.
When the Bombers confronted Collingwood on Anzac Day, James Hird had decided he did not want O'Brien as the plus-one player. But he also had a problem in the midfield where his initial match-ups - Sam Lonergan on Dane Swan and Heath Hocking on Scott Pendlebury - were not holding.
Hird chose to post an extra man in the midfield, but he sent Angus Monfries to negate O'Brien. It still left a spare man in Collingwood's back half but at least it was not O'Brien, who is damaging with his run and carry.
This is part of the problem for coaches. A coach can send an opponent to pick up the spare player, as Fremantle's Ross Lyon did with Sam Fisher against St Kilda a few weeks ago, but that will create another free player. In that case, Jarryn Geary ran loose for a lot of the game but Lyon would have taken that as a preferred option.
The only way you can stop it definitively, is to go man-on-man all over the ground. But the days of the six-man forward grid against six backs with the same numbers across the midfield have largely gone. Coaches demand the right to send their players where they want them and few of them want more than four or five permanent forwards, giving them licence.
As David King pointed out on AFL Insider this week, there are smart ways to handle it. Hawthorn kept trying to get Luke Hodge free against Sydney in Launceston last Sunday, allowing the captain to act as ''quarterback'' as he does so brilliantly. But Sam Reid kept sliding off his opponent, Ryan Schoenmakers, and going with Hodge when this happened. Sydney's intent was clear: if Hawthorn was to have a plus-one, it was not going to be Hodge.
One well-regarded strategist and former AFL player told The Age this week that plus-one strategies had become a ''huge'' part of the vernacular in coaching.
''There's a lot more science to it now,'' he said. ''What you're looking at is: who is it? Where do you want him? What's his role? That's been around for a while, but it was pretty random before. We only used a loose man when we were under scoreboard pressure.''
The reasoning changes week to week and depends on the opponent. ''It can save you goals against an aggressive side that wants a shootout. Against the slower, more mechanical sides like Sydney and Hawthorn, it's not so effective. The good sides control who that plus-one is; the bad sides can't control it and the players can't get it right under the pressure of an AFL game.''
It does not seem so long ago that the players were nominated in positions on Thursday nights and appeared in the media that way - with six on six in each half and the rest in the middle - and mostly that is how they lined up on match day. Not any more.