AIS - Active Isolated Stretching

In a nutshell, A.I. falls somewhere between the slow, static, yoga-like stretches most runners practice (sometimes) before or after they run and the ballistic leg swings and kicks elite athletes are often seen performing before they compete.
The underlying theory behind A.I. is that if a muscle is stretched too far, too fast, or for too long, it elicits a protective action known as the myotatic reflex, causing it to automatically and ballistically recoil in an attempt to prevent the muscle from tearing. This occurs about three seconds into a stretch.
Therefore, A.I. practitioners hold a stretch for only a second or two, before the myotatic reflex kicks in, then relax and repeat 10 times. Using this technique, the muscles exhibit a greater range of motion over the course of each set of stretching repeats.