PNF is a very powerful stretching technique, originally used by Eastern European gymnasts.  It utilizes both stretching and mobilizing the surrounding muscles to improve the strength and flexibility of the tight joint.  PNF stretching is reviewed by James Scifers in the article "The Truth about PNF Techniques" in Advance magazine.  

PNF stands for proprioceptive neuromuscular facilitation and works to use more functional movements to improve range of motion.  "PNF stretching is superior to other stretching techniques."

However, this is a very powerful technique and any user should consult with their doctor or therapist before pursuing it on their own.  PNF stretching can fatigue the area and increase the risk of injury from a sudden extension of the joint after the stretching session. 

This article covers a few of the PNF techniques, including Hold-Relax, Hold-Relax with Agonist Contraction (or CRAC ), and Agonist contraction; as well as strengthening techniques. 

See the article here:  The Truth About PNF Techniques