PNF

Developed as a form of rehabilitation, Proprioceptive Neuromuscular Facilitation (PNF) is a more advanced form of flexibility training that involves both the stretching and contraction of the muscle group being targeted.

All PNF stretching requires is that you stretch a muscle and then forcefully contract that muscle before stretching it again. As you move into the stretch after the contraction, you should be able to stretch further than you did before. This allows you to create more length in the muscle and receive a greater flexibility benefit from the stretch.