Wave drills in volleyball are quite useful when you work with larger groups of players. They’re also good in high intensity drills. They can help avoid excess fatigue.
Basically, wave drills involve grouping players. You then rotate them through positions in a game or drill together. Doing so can effectively minimize down time in the form of players sitting out. You can also use them to move players into a less demanding role after a high intensity sequence. For example, MBs shift from fast-paced front row play to serving or defense.
A game like Winners 3s is a simple version of a wave structure. At the end of each point, one group of players comes off while another group comes on. A third group may also shift from the challenge side of the court to the winners’ side.
Another variation on this is breaking the team in to cohorts of three. They then play a 6 v 6 game during which those cohorts are rotated through front and back court positions. For example, a new wave comes on in the back court position on one side after each rally ends. That then cascades the waves through. It pushes the back court cohort on the other side of the net off as the front court group moves into their place. This allows you to have players on for 4 straight rotations. They are only off a minimal amount of time (1 rotation if you have 5 groups, 2 rotations if you have 6, etc.).
You can also wave on errors. Say you have 18 players. You split them into six groups of three. Three teams are assigned to each side of the court. Two teams are on and one is off waiting. The teams play through a rally. One of the cohorts on the losing side is replaced by the cohort waiting on the sideline based on some rule, like which group was at fault for the point lost.
I’m sure you can think of numerous other waves ideas. In fact, you probably use them in an ad hoc way right now. When you flip front and back row during a drill or game (like in Bingo-Bango-Bongo after a big point), that’s a form of a wave. The advantage of formal the wave rotations, however, is players are responsible for automatic waving. That means you don’t have to stop things to do it. This saves time and keeps the training intensity up.