Hi Etherfish,
It will not work in stretching the tongue, but will probably work in the milking stage.
What Bob Cooley wrote is (mainly) the contraction/relaxation (CR)method, though he may used different names. An effective way of stretching, of course. (But, btw, I don't think he is the first one to come up with that. I found a couple of books talking about this. But Bob Cooley seems to be a better advertiser.)
In the usual muscles, what we think we're stretching are the tendons which connect the muscles and bones (the "lever") together. But the length of the muscles is actually more crucial for flexibility. As our muscles get leaner or more relaxed, it becomes longer, and we become more flexible. That's why even in a single day, you may find your flexibility changes. That's because you tends to have tighter muscles sometimes, e.g. in the morning. So, what we're working on, is more the muscles than the tendon. If we are able to relax more the muscles we're stretching, we will find our flexibility improved much both immediately and in the long run. The CR method works well because it makes use of the information from stretching (usually consists of slight pain or discomfort in the muscles you're stretching), helps you build up awareness or that muscles (by feeling and then tightening them); and the "relax" part will then be more effective. Probably (this part is my guess) it also works by engaging the muscles actively, so that the rearrangement/repairing of muscles tissues is faster after the practice.
The difference here, for the tongue, is that the fiber (in the frenum) we want to "stretch" is NOT attached to the muscles of the tongue. So contracting the tongue against the stretch will not have the same effects here.
However, I think it's a nice idea to do it in milking, where we literally want the tongue muscles (rather than the fibers in the frenum) to be longer.
Alvin