Proprioceptive Neuromuscular Facilitation (PNF) is an advanced form of flexibility training that involves both the stretching and contraction of the muscle group being targeted and was originally developed as a form of rehabilitation. It is also effective for targeting specific muscle groups and improving muscular strength. PNF stretches may put added stress on the targeted muscle group, increasing the risk of soft tissue injury. To help reduce this risk, it is important to include a conditioning phase before a maximum, or intense effort is used.
(Source: The Stretching Institute, www.thestretchinghandbook.com)