Thanks for the insight! Maybe I'm trying to hard to fight the recoil. I just picked up this 1911, and I've been shooting a Sig P229 in .40 for the last couple thousand rounds.
I also did a little bit of shooting with just my thumb and forefinger that day as well, which was more accurate than my two handed shooting.... I learned it at a recent Dave Harrington class I took to work on trigger control. Beyond the support hand slipping off, I have a trigger control problem. I've started dry firing this week, as everyone seems to recommend it to improve. I just want to make sure I'm practicing the correct grip while I'm ingraining habits.
http://img266.imageshack.us/i/shootingwiththumbandfor.mp4/
Also, here is the a view from the alternate side.
http://img695.imageshack.us/i/shootinghandcomingoffof.mp4/
I don't think it's a finger length issue at all. Two things I see right off. One, you're stopping the gun when it gets to the top of it's rise, releasing the tension on your support hand (watch the lower fingers as it's almost a ripple upwards), adjusting/regaining your grip and then bringing the gun down to normal index on the target.
If you're fighting the gun to try and stop it at the top, you're almost pushing forward with the support hand making it easy for your hand to want to release....it's going in that direction already. If we're shooting fast splits we're going to be pushing forward to drive the gun (after ignition), but that is happening with the strong hand, not the support hand, which is clamping side to side to make the gun track straight up and down and balance out the whole system.
Try shooting a couple of shots with a very relaxed grip and just let the gun go up and down almost by itself...it's not going anywhere. I'll often shoot a gun (as a demonstration) using only my strong thumb, middle finger and maybe ring finger in a very loose grip...partially putting my body off to the side so it's clear I'm not offering much resistance to the recoil/flip. The gun flips a lot more, and moves my hand, but it still comes back down pointed generally at the target. Once you see it come back quickly, by itself, that's something to strive for when you're shooting normally.
In a similar fashion, if you shot a .22 your support hand probably isn't doing the same thing...because you're not trying to control it since you know it's not going to move much to start with. I'm sure others will have more, better ideas, but that might be a start. R,