You may recall going to your doctor to get a physical or check up and they used a funny looking little rubber hammer to hit your knee. When they hit the right spot on the front of your knee, you kick your foot up into the air. When they do this, they are checking your stretch reflex.
As a protective mechanism by the body, when your muscles feel a sudden stretch, they contract as a reflex in order to better protect the joint that is involved with the activity. In the example given here, your quad experiences a stretch when your knee is hit which causes the muscles of the upper leg to contract in order to protect the knee.
It may not seem like it is very important, but the fact is, training the stretch reflex is critical for improving jumping and power production, because the greater the stretch placed on the muscle, the greater the contraction. This is the basis of plyometric training. You may have hear plyometrics as a key for improving your jumping performance.
When you play a game of basketball, at some point you will be jumping for a loose rebound. As you land from the jump, your glutes, quads, and calf muscles are stretched in order to decelerate your body weight. In an effort to protect the ankles, knees, and hips, your body will contract the very muscles that were just stretched. As they contract, your body is once again thrust into the air. For the most part, the greater the stretch, the greater the contraction force. And, the greater the ability to jump. Refining and training the stretch reflex is vital for speed and power athletes.