From my research lab that I'm part of.... There used to be a rule where in practice they would hit the ball over the net, and then run underneath the net to go get their ball, meanwhile the person behind them in line would also spike the ball down. That resulted in concussions, being hit in the back of the head with the ball, so there was a policy change last year, I believe, where now they're no longer allowed to run under the net. They have to stop and run around the net to go get their ball to try to reduce concussions.
One of my friends, who's the main researcher on that, has said that he's been hit in the head trying to get his ball.