Once the #1 player's fatigue hits x, x being what you set the fatigue limit at, then the next player at that position will take his place
Fatigue limit = 30
RB 1 fatigue 32
RB 2 fatigue 14
RB 3 fatigue 0
It'll use RB 2 on the play
If the next play RB 1 has 29 fatigue, it'll use the RB 1
RB3 will only get used if RB1 and RB2 are above 30
Meaning its almost parabolic or exponentially fewer plays the further down the depth chart you go, as multiple conditions need to be satisfied to move down another spot on the depth chart