Technically it's called a Golgi Tendon Organ that monitors the tension on a tendon and will release the muscle further if it is properly triggered to do so. The contract- relax MET (muscle energy technique) of which PNP is one of is a function of the nervous system. It functions via whats called reciprical inhabition. A simple example: your triceps has to relax an allow extension for your biceps to contract and shorten.

All of this should be done slowly and gently to work best: this is not an aggressive technique. Hold your position gently at the "barrier" before engaging contraction. What I mean by barrier is that moment in the range of motion where you can just start to feel tension through the tissue. You don't need to force them into the technique; technically this is not a stretch. Use the technique as described, asking them to move deeper into the range of motion to increase the effect of reciprical inhabition. Stretch them a little further with the last set. It is better to use multiple small releases to accomplish the release then pushing one big one.

You can do this before or after mysofascial release/deep tissue massage to release the nervous system to allow the tissue to lengthen. You can think of PNF as getting the little guy in the back of the brain (nervous system) to flip the levers that allow the hamstring to lengthen once your have given the tissue the physical ability to do so by working it directly. Best results are achieved by a combination of techniques. To be sure, there is much more you can do, and you can get really really focused with MET techniques releasing isolating fibers instead of being very general once your sensitivity is there.

Good rehab therapy is to also rehab the hamstring by doing lengthening contraction (eccentric contraction) to restrengthen the torn fascial structure. You do this by slowly controlling the descent of a weight on a hamstring curl 15-30 seconds per exercise, say 10-15 lbs. 15 times 3 sets. This will help prevent re-injury.