How does the player substitution work at this level?
That is a simple question with a pretty complicated answer. I'll try not to geek out too badly, but here goes:
At its most basic, it is tall girls in the front row slamming the ball to the floor and short girls in the back keeping the other team's tall girl slams off the floor.
More detailed - six players on the floor at a time, six positions they are occupying; every time the serve changes from one team to the other, the team about to serve must rotate their players from their current position to the 'next' position. That means that every time the players rotate, one tall girl in front is rotating to the back while one short girl in back is rotating to the front. And that is when and why most of the substitution happens; to replace tall girls with short girls and short girls with tall girls.
Complication #1 - there is a limited number of substitutions per set. 15 in NCAA volleyball. That limit isn't all that 'bad' and teams don't often actually bump up against it, but that is because they go into a set having planned for it. And occasionally, teams do bump up against it. In last night's first set that went 'extra points', Wisconsin essentially ran out (had one left but had already changed their substitution pattern because they were so close and were saving that last one with a specific purpose in mind).
Complication #2 - not all substitutions count against that limit. And that's where we talk about the girl in the opposite colored jersey. She is called the 'libero' (I'm guessing Italian for "the free one", or something like that). The libero can sub in for two players and positions instead of only one (but with one minor restriction not worth going into here); other players are limited to one of the six positions; that is, short girl #1 subs in at the postion occupied by tall girl #1 the entire set. An alternate short girl #1 can come in at that position, as can an alternate tall girl. But someone who has 'been' short/tall #1 can't then come in at short/tall #2. At least not in the same set. The libero coming an going with the two 'middles' doesn't count against the limit; they are 'free'. (The libero does have other limits, but she is 'free' when it comes to the substitution count limit.)
Pretty much every team uses the libero to sub in for 'tall girl in the middle of the front row' (the middle blocker) because most of the time she is the tallest one and likely the one least capable of handling the 'keep the ball off the floor' back row duties. After that, the substitution 'pattern' is put in place by the coaches with the '15' limit in mind. If a team has a player who is tall enough to play in the front row but agile enough to handle back row duties, they will play that player through all six position rotations without every subbing her out. She's a "six rotation player" who "plays six around", thus saving on the substitution limit for other positions and players and delaying the point in time when the limit might be hit. Six rotation players are quite 'valuable'.
And after that, the ways and variations on how coaches come up with to vary the substitution patterns and use their 15 subs are myriad. The discussion Arlen and I were having has it is basis the choices the Minnesota coach made and could have made regarding his substitution pattern and use of his 15 subs in the match against Wisconsin to work around players missing due to injury and COVID, etc.
Questions welcomed; corrections welcomed. I hope that helps. (Watching to understand what a team is doing with their substitution pattern is a whole lot easier in person than it is on TV!)
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(Adding 'short" and "tall' being relative terms here. The "short" girls are probably in the 5'8" to 5'10" range at the level we're talking about here, with "tall' being above and maybe well above 6 feet. But "short" can still mean short; I remember a half-dozen years ago Indiana had a girl who was listed at 4 foot 11 inches, and she actually played back row for them.)