I tried some when I was a kid, which I think amounted to breathing exercises. Probably some older theories behind it. Tried some later around the onset of my twenties (I think), but there wasn't much in it for me, since I was quite reflected about my stutter, and I didn't really benefit from the techniques used. But I know it's helped others (who are worse off than me, we could say). There are various ways of doing it. The trick is finding the right one, I guess.