What does the act of stretching actually do?

It relaxes collagen: Stretching has several benefits before activity including taking joints through their full range of motion, and increasing blood flow. Most importantly stretching actually unwinds collagen fibers within muscles, ligaments, and tendons. The analogy would be silly putty. When you pull it slowly it stretches out. When the collagen unwinds and stretches we call it viscoelastic relaxation.

Prevent injury: Loosens tight muscle fibers to prevent injury.

Stretching: Used to maintain flexibility and muscle balance. Not used just prior to competition because of some concern that it may effect power output. Dynamic warmup now use prior to play. Believe is that flexibility provides better range of motion and therefore improves muscle contractile properties through a wider range. Controversial as studies are mixed about preventing injury.