Behind closed doors - in match committee discussions and amidst the organised chaos of a coach's box on match day - the hidden impact of the new sub rule is being played out right now.

Hawthorn coach Alastair Clarkson raised the issue of time when asked about the sub rule by Leigh Matthews last week on AFL.com.au's Talking Footy program.

Clarkson said the new rule continued to be a challenge for coaches as they spent extra time making weekly judgments working out who the substitute would be (so far this year the Hawks have used a variety of players as subs - Luke Bruest, Shane Savage, Xavier Ellis, Brent Guerra, Jordan Lewis, Cameron Bruce and Brendan Whitecross).

With match committee meetings already filling a huge block of time in the coaches' week, this additional impost is no small matter. Adding complexity to the selection process inevitably adds to the strain and the hours in meetings. 

Geelong coach Chris Scott said after the club's win over Collingwood that the Cats looked at the sub as a specialist position. "We never use our 22nd player picked as a sub," he said. "It's a specialist position."

Their sub that night Allen Christensen kicked a handy goal late in the third quarter after coming on to replace the injured skipper Cameron Ling. Christensen is beginning to grab hold of that 'specialist position'. He has worn the green vest four times before coming on while Geelong has twice used veteran Darren Milburn as a sub.

To appreciate the problems this 'specialist position' selection can cause, you need to understand the layers of complexity that go into the apparently simple process of selection every week.

A team has a pool of players good enough to warrant senior selection and needs to consider each player's capabilities, their suitability against specific opposition and, critically, their ability to handle the physical load of an AFL game. The most selection time has always been spent on the last player to be picked. Double that for discussion on the substitute.
 
Choosing who fills the slot is obviously more difficult than the extra interchange. Although conventional wisdom suggests clubs look to a flexible player as their sub, another line of thinking is that if a team has a large core of flexible players within its 21, does that change the type of player it chooses to be the substitute? Although it is unlikely it will ever be a designated ruckman, what is proving most likely is that it will be a young player, someone who has played fewer than 25 games.

Whereas clubs previously graduated young players with talent to move to senior ranks by playing them between 60 - 70 percent game time, under the sub rule at least 20 players will play 80 percent plus game time. Some clubs are reluctant to risk selecting young players in the 21 for fear of burning them out. If you think a difference of ten percent is nothing special, then ask yourself why clubs exult in 'one percenters'. Every minute of extra time played adds higher percentages in training loads to prepare the body to cope. It's new ground for conditioners.

Just over half the players chosen as substitutes in 2011 have been players in this fewer than 25 games category. Last week 14 of the 16 substitutes chosen had played fewer than 25 games. The exceptions were Collingwood's Brad Dick (on the edge with 27 games) and Carlton's Dennis Armfield (49 games over four seasons).

Young players are still being picked in the 22. The number of players making their debut is consistent with previous years (once the Gold Coast factor is taken out it's 48 newcomers after eight rounds this season compared to 51 in 2010 and 49 in 2009). Those 48 have played 190 games (average 3.95 games per player) in 2011 as opposed to 209 (4.09 average) in 2010 and 197 (4.02 average) in 2009.

None of the young substitutes has been a back-up ruckman. One issue occupying coach's minds is how to develop young ruckmen without exposure at senior level so they can succeed the number one choice when the time comes, or injury strikes (Hawthorn's scenario on Sunday against Sydney). This fundamental by-product of the rule has generally been overlooked in commentary. 

As with most things to do with coaching and list management these days, this type of analysis will have happened through the week - strategic change under the pressure of match day is rarely applied in the modern game, as the role of the coach is to reinforce, rather than to throw bombs into well-drilled systems. That's what players mean when they say things like "we stuck to our structures" or "followed the plan."

But the sub rule has introduced a new layer of pressure into that already ballistic environment. The choice as to when to use the sub can never be pre-planned.  "You're trying to watch the game and see it all evolve and a bloke comes off with what might be a minor injury and you're forever sitting in the box trying to get a medical report and you're taking your eye off the game working out whether you will activate the sub or not," said Clarkson on afl.com.au.

To understand the impact of that description it is necessary to put yourself into the coach's seat on game day. The coach is determining who is moving into the right position to complete the zone, whether a forward should be thrown into the ruck, whether an extra player is needed at the stoppage or behind the ball and making such calls based on advice coming from all angles - his assistants, the conditioning coach and, occasionally, the player himself (a key reason why more coaches are working from the bench).

Even before the substitute rule was implemented there was an urgency to find out whether an injured player was off for the game or not because of the impact it might have on rotations and whether a positional move was necessary. Multiply that urgency by 100 now that a player once subbed is finished for the game, fit or not.

Scott was hard on himself last week when he said replacing Ling with Christensen during the third quarter was a coaching error. Ling, who looked as though he was carrying an arm that was about to drop off when he left the ground, was not as badly injured as first thought.

By the time everyone realised his injury was not bad enough to keep him on the sidelines the coach had made the call, and a cheerful Ling had been subbed from the game. "It was around the time we wanted to make the substitution anyway," said Scott. His decision was well and truly verified this week, with the news that Ling is missing from Friday's match against Carlton.

Wasting time waiting - tossing up pros and cons of change - is barely an option. Clarkson laughed at the thought of his good mate and former assistant Richmond coach Damien Hardwick waiting 20 minutes before subbing Jack Riewoldt off the ground in round 2.

That meant the Tigers had 20 minutes with just 20 players in the game. "It becomes very demanding and usually you would say 'that is it … if we can't get an assessment on Jack (the injured player) that is positive for us then let's bring on a fresh runner,'" said Clarkson.

The difficulty of the decision increases if the game moves past the midway point of the third quarter and there has been no injury.

Scott admitted Ling was on a list of about 12 players that were being considered as 'to be subbed' candidates before the skipper hurt his shoulder. That is a long list. Coaches don't close their eyes and play pin the red shirt on the sub. They would agonise in their own mind about what decision to make, before, during and, sometimes, after.

Don't be surprised in this era of assistants managing mids, backs, forwards, talls, shorts, physical capacity, we have a new man for a new job in 2012 - a coach to manage the bench and the permutations being introduced to the game on and off the field through the sub rule.