Proprioceptive Neuromuscular Facilitation, or PNF, is a stretching technique that uses various combinations of stretching, contracting and relaxing of the muscle group. It is a very effective way of increasing flexibility.
By contracting the muscle, you fatigue the muscle fibers slightly so they are less likely to contract in resistance to a second stretch.
The most common method of PNF uses a contraction and relaxation of the muscle you are trying to stretch. Some methods also use a contraction of the opposing muscle in order to force relaxation of the muscle you are trying to stretch.
PNF is often done with a partner and is especially effective that way but not necessary. Your partner would provide resistance against an isometric contraction of the muscle (that's contracting the muscle without moving the joint) and assist you in moving back into a passive stretch.
If you choose to practice with a partner, it's important to begin slowly and always communicate to avoid injury.
There are several different methods of PNF but the most common technique is the contract - relax method.
You should be able to stretch a little further during the second stretch.
By repeating this 1 to 2 times, the muscle is stretched a little further each time increasing the range of motion.
If working with a partner, you would isometrically contract the muscle against your partner's resistance and as you release the contraction into the "relax" step your partner would help move the muscle into the further, second stretch.
If you're new to Proprioceptive Neuromuscular Facilitation, stick with once a week sessions for a while since this technique is a little more intense than passive stretching.
If and when you work up to more frequent sessions be sure to leave at least one day of rest in between.