This is a great summary from an article Bandha Yoga – The Scientific Keys
“Sports medicine experts long ago perceived that this particular reflex arc could be carefully manipulated to lengthen muscles. Using this knowledge, they invented a technique called proprioceptive neuromuscular facilitation (PNF), or facilitated stretching. It is the most powerful method for gaining length in muscles to improve flexibility. Yoga uses stretching, so why not use PNF in our practice to deepen the asanas?
It is important to remember that any powerful tool, including yoga itself, is a double-edged sword (like a surgeon’s scalpel). If used carelessly, it can cause injury. This is also true of facilitated stretching. The key to using techniques like this is to apply them slowly and with care. They are like a tincture of medicine, so use less muscular force rather than more.
Facilitated stretching works as follows: after warming up, we take the target muscle into a moderate stretch. This establishes the muscle’s “set length”—a measure in the brain of how far the muscle can lengthen. Stretching a muscle produces tension at the muscle-tendon junction and stimulates the Golgi tendon organs located there. The key to PNF is to then gently contract the same muscle that we are stretching. This combines the biomechanical event of positioning the body into a stretch and the physiological event of intentionally contracting the stretching muscle to amplify the tension at the muscle-tendon junction. The Golgi tendon organs fire more intensively, producing a powerful relaxation response. We then stop contracting the target muscle and “take up the slack” by going deeper into the stretch. The net effect is a new set length.”