I shoot sporting regularly and have for 7-8 years. About three years ago I came down with this precise affliction. Like a cold-solder connection, as Jim described it. It got so bad that I would flinch 7-8 times per round (100 birds). I blamed it on the trigger first, then pre-mounting, then other things. Never really figured out why it started or how to stop it. A few months ago I received Bill McGuire's new video, Focus and Fire. I know Bill well and was eager to watch it. I so identified with the style of shooting sporting clays that he teaches that I immediately put it into practice. My scores have gone up dramatically, won my class at the QU shoot last month, and now for the kicker............ my flinch is gone!! It was exactly what jakeroo described. Too much gun movement in tracking the bird. Now, I insert the gun on line at a predetermined point along it's trajectory and look back for the bird, tracking it with my eyes while it approaches my muzzle. Then, just before it gets to my muzzle I pull out front with a very, very short move and shoot. I think it was the anticipation of the shot that caused the lock-up. Not anticipation of recoil in my case, as I actually went from 1 oz. loads to 1 1/8 oz. at the same time I tried the new method.
I sympathize with anyone who suffers with this malady, and it truly is suffering to a serious shooter. Never flinched on a live bird in my life, only clay ones, and never pulled the trigger on a flinch. Hope this helps someone. Stan