How tired the player get towards the end of the game. But honestly, the game admin/developers have not ever stated the effects of stamina, (or any of the other skills) so it is a guessing game.

Chidash wrote: 2. exactly what does stamina effect? Does it reduce all skills (eg passing etc) or just the other physical skills speed, strength and agilty. Or does it only work on reducing the players' energy? It reduces the overall player's performance. So his overall rating for his position. I think I've seen it written that stamina really kicks in during the fourth quarter, but the jury is still very much out on the effects. Several top players have told me that they have never trained stamina.

You can train stamina and put your players in a position to perform consistently through 4 quarters
OR you can train other skills higher and your players will be in the best shape to perform in Qtr1 after which their efficacy gets reduced linearly. The reevaluation is done not after each play but after time slab x (I guess x = 5 like in teamhb)