Ever wondered why your favourite player keeps getting substituted?

There are levels to it.

Ever throw your hands in the air in pure exasperation as your favourite player get's hauled ashore just as the game is in the melting pot? Or make a bold claim that your man is going to make the difference after the break only to see him benched and in those snug three-quarter lengths? On occasions, the coach makes a gut decision or changes tactics to suit the scoreline. More often than not, and it is an increasing trend in modern rugby, the replacements have been pre-determined. Some players are told in advance of games that they will be coming off after 45 or 50 minutes and to expend their energies to suit. Ronan O'Gara had two gripes when he was a player, at the height of his form and influence. Substitutions and player rotation. As his career progressed, he began to accept that he could not play every minute but he did like to have a say in every game possible. The Racing 92 defence coach tells us:

"[Rotating players] is something that you need to do and it's important but, as a player it's something I did not get. "It wasn't done that much of Munster. I would have been playing with a lot of guys that [played regularly]."

The former Ireland outhalf is now in his fifth season as a coach, in France's Top 14, and believes some of the greatest coaches manage their big squads with a process that makes it nigh on seamless. "There's great skill in that," he says.

"They watch from a distance and monitor the GPS with the GPS man, who is crucial in any organisation. That's the way the game is going. "In every team I was involved with, the head of fitness was on nearly the same level as the head coach. They'd be tight with the manager and it'd be, 'This is the amount of minutes you're allowed on the pitch today'. "So you've a final play [in training] and the play does not go well, the head of fitness comes on, 'Right, get off the pitch!' "While in an awful lot of other teams you hear 'We'll do that again'. That creeps in to other aspects of the game - concentration, performance, dropping balls becomes the norm. So you go six minutes over in the session to get it right. "The following week, you go eight minutes. Then you just lose your whole intensity philosophy, about why you train with intensity."

Jonny Davis [pictured above with Les Kiss] is Head of Fitness up at Ulster Rugby. While he may demur at the suggestion, he does admit his role, and that of his team, is key in the modern game. When it comes to team selection and substitutions during games, Davis offers an insight to the process.

"It's a collaborative aspect, between strength and conditioning, physical therapy and performance medicine, in terms of what we know about the player and what we know of the positional demands. That's where we have that weight of evidence - this is what he's been able to achieve to date and this is what he needs to achieve in order to perform. "If the player is way off in terms of percentages - where they are to where they should be - then you ask the question. "Right, if it was a final it's no holds barred. You've got to get your best players on the pitch, nine times out of 10 and bearing in mind that they don't have a significant injury that will really impede their performance. "Everybody here is hugely respectful of each other and of that huge weight of evidence. It can't be a pissing contest, at times. Everything's got to be objective and you back it up in collaboration with physical therapy or Dr Michael Webb, our medical director, and other, third-party consultants."

The performance level metrics are all fed into a data-base that then gets live data, from the GPS units each player has stitched into their jerseys, as the match takes place. Davis adds, "You weight it all up and hopefully it's the right decision... all of the time."