jm,
I actually read up on that in a book about stretching years ago. The old style stretching of hamstrings was to “hang and bounce”, right? According to the book, inside the tendons there are tiny receptors that sense the tension inside the tendons. If there is a successive increase, the receptors signal “Everything under control”. If the increase in tension is sudden and forceful, the receptors signal “Too much, too quick” and the tendons TENSE UP from that! The idea is that a stiffer tendon will not get accidentally pulled out of shape (as easily) and so they go all stiff and immovable. That means you are “bouncing” off your muscles, and the next step is ripping muscle tissue.
I suppose this is a good idea (from an evolutionary point of view), since muscle damage heals quicker than tendons do.
As for dynamic stretching - within the range of “Everything under control”, I imagine that this is slower and more successive increase / decrease, more like a sinus curve rather than a heart curve. In reality, that is likely to be what happens to the tunica when you jelq. I find that more likable than sharp, sudden clamping, which is more like the bouncing mentioned.
regards,