I bought the book too, and Deetron-- you're not the only one with a difficult time trying to contract and stretch at the same time! Plus the book doesn't explain how quickly/slowly to move, and I was 1)afraid to go to quickly and 2)therefore moving so slowly I was tiring out before I got many stretches in. However, I mentally changed how I was doing it and I have seem some improvement in my flexibility in just a few days. The concept I got from this book is it is supposed to be a very natural-feeling movement and not painful-- so I thought about the way I stretch when I get up in the morning. I put my arms up in the air and stretch, but I also contract my muscles and move around at the same time. It feels great, but it's not the same "lengthening" stretch we all do during exercises. So I applied that concept to how I was stretching my legs, and not only did it feel really good, it did help me deepen my stretch. (I stood in a wide straddle stance, tightened the back of my legs and actually *yawned* as though I had just woken up, lol, and started to lean forward as I did so.) The only problem is, I'm not sure how to apply this to Cooley's positions, because so many of them just don't feel "natural" and therefor are difficult to do. I was actually discussing this book with my dad, and his doctor has him working on a similar concept to correct body posture-- the whole idea is that it isn't natural for our bodies to sit in one position and hold it while we stretch, so we can see faster and better results by moving and stretching at the same time.
As far as PNF stretching, there are two differences I see. One is that you are not doing a full range of movement while you perform either the contractions nor the stretch that follows. The second one, and I hope I remember this correctly from school, is that proprioceptive neuromuscular facilitation (meaning your perception of your body's position in relation to how your nerves and muscles are working together) works more or less by helping you to relax. We have a hard time going into stretches because manyof us find it difficult to fully relax our muscles. Contracting a muscle as hard as you can, and then relaxing/stretching it immediately after teaches your body and brain the difference between tightening and relaxing, allowing for more relaxed muscles and a deeper stretch. It probably does some other things too, I'm just saying that's what I remember from school 🙂