Proprioceptive Neuromuscular Facilitation (PNF) is a more advanced form of flexibility training that involves both a stretch and a contraction of the muscle group being targeted. PNF stretches were originally developed as a form of rehabilitation, and to that effect, it is very effective.
The purpose of PNF is to evoke motor responses and improve neuromuscular control and function. Putting a muscle in a stretched position (passive stretch) and holding for a few seconds. Relaxing the stretch, and then stretching again. This is a technique to achieve maximum static flexibility.
PNF stretching has been proven to improve active and passive range of motion. It can be used to supplement daily, static stretching and has been shown to help athletes improve performance and make speedy gains in range of motion. Not only does it increase flexibility, but it can also improve muscular strength.