What a lot of people don't understand about volleyball is you don't have free substitutions like in basketball. A player must rotate through six positions on the floor, and most of the players play only the front three rotations or back three rotations (most outside hitters like Lund play all 6). In Pitt's current rotation, Ndee plays in the front row for three turns while Fairbanks plays in the back row. Then Ndee subs out from the RS position and Fairbanks takes it over, while Akeo subs in as server in the back row. After three more rotations, Fairbanks goes back to serve and Ndee subs back in. You aren't allowed to play Ndee only in the front row for the whole match.
Think of it like a batting order in baseball. You can't just send up your cleanup hitter whenever you want. After you use him once, you can't use him again until you go through the entire lineup.
Pitt uses something called a 6-2 rotation which means they are relying on two different setters across the six rotations. Before the Fairbanks Experiment, every three rotations you'd see Pitt sub out a RS (Ndee) and a setter (Levers) and sub in a RS (Lockwood) and a setter (Akeo). Likewise, you always have one MB in the front row, so when Nwokolo subs out, Gray subs in. Gray only plays in the back row when she is serving, because she is a far better server than most MBs.
There's a finite number of subs you are allowed in each set, so every once in a while you will see a coach forced to have his front row players serve out of desperation. In one of the Baylor-Texas matches this year, Baylor ran out of subs and was forced to play several points with the same six players on the floor until the set ended.