I'm sure there are variations but the one I read used the following
For example if you are stretching quads:
Position yourself so that you feel a decent stretch in the quad.
Slowly contract the quad(try to extend the knee) for 4 seconds, then for 6 seconds keep contracting as hard as you can. Obviously you should stay at the same position, which is hard to do unless you got something to resist your attempt to extend the knee.
Then rest for 2 seconds,
Then try to stretch the quad even further, you should be able to. Then repeat the whole thing 3-4 times.
Takes a total of roughly 1 minute for every stretch.