Brain and Body
Reaction Time can be broken down into multiple steps. First your eyes receive a signal, next your brain computes that signal, then your brain sends more signals to the parts of your body that need to move. Sports Vision Training can have a large impact on Reaction Time. The brain takes into account situation, visual input and muscle memory to react in game.
If we take Volleyball, for example, and look at the Libero specifically. The Libero's sole job is based on Reaction Time. When they prepare to react to a Hitter's spike there are really only 3-4 directions the Hitter can send it (situation). 25% of spikes are not going to be returned, but the other 75% have a chance to be saved. Based on where the ball is set (visual input), the Hitter can try to go around the wall or through it. When the Libero adjusts to the information the brain is processing, they can then get their arms and body in a position to have the most amount of success (muscle memory).