If you're hitting top-left Charlie-Delta as a left handed shooter, you're not thumbing but more likely heeling or anticipating recoil.
In this case, it's more likely heeling, ie flexing the muscles at the heel of your palm too much when pulling the trigger on presentation of your sidearm. You could also be pushing with your hand too much as you present your pistol to sight.
Slow down your motion and do it all in a slow yet smooth draw. Practice it and slowly increase the speed. Take mine of your hand and its movements. It stays still through the draw and trigger pull. The only thing moving should be your trigger pull, and with that, the movement is just a smooth pull to the rear with the tip of your index finger until the shot breaks.
Last edited by ILLusion; September 10th, 2008 at 02:34..