PNF Stretching, otherwise known as Proprioceptive Neuromuscular Facilitation, was first developed in the 1940's by Dr. Herman Kabat, a neurophysiologist and physician, as a means of working with polio patients.  His work was influenced by Sister Elizabeth Kenney, an Australian nurse who devised a method of helping them loosen their muscles and free up their bodies by stretching and strengthening.  Dr. Kabat later teamed with Henry Kaiser to establish the Kaiser-Kabat Institute in Washington, D.C.     After opening another location in Vallejo, CA in 1948 and teaming with physical therapist Maggie Knot, they began to train clinicians from around the world in this remarkable technique.  

What is it?  
It is a hands on method of alternatively stretching and strengthening your muscles, causing them to release excess tone in the muscle that may be inhibiting correct neurological function. 

How it is done? 
The technique is described as "contract/release" stretching, in that you allow a practitioner to put your body in a position to be stretched, then alternatively contract the muscle to tighten it, and then release it to allow it to stretch. 

Does it hurt? 
No more than the average stretch might hurt, but even if there is momentary discomfort, it is alleviated as soon as you release the stretch.  

How quickly does it work? 
Almost instantly.  The act of quickly alternating between muscle contraction with muscle lengthening decreases the ability of the muscle to hold "tone" in it, so the muscle is relaxed and returns back to it's normal length. 

How often should you do PNF stretching? 
As often as you want, although if the same muscles keep getting tight over and over, you probably want to do some NKT to see if there are any underlying mal-adaptive muscle firing patterns that need to be addressed.