I'm a firearms instructor for a large PD and I was on SWAT for 8 years. I used to be fairly competitive in USPSA and 3-Gun for a while until kids came along. I tried the finger on the trigger guard thing for about 2 years. Eric Grauffel, a French IPSC shooter, still uses this grip (See: http://www.ipsc.org/image/eric03.jpg
) but most people have abandoned it, including me.
The biggest problems with putting the finger on the trigger guard is that it takes away from the strength of your support hand; which we know for a fact is equally (and some will argue more) important as the dominant hand for stabilizing the pistol in the two handed grip. Secondly, the finger will tend to "steer" the pistol back and forth and it can lead to poor control, especially on multiple targets. For me, the finger on the front of the trigger guard meant that my whole left hand was shifted forward, leaving a gap between the backs of my hands (i.e. the heels). My weaker support grip became problematic when my hands were sweaty or when I was getting tired. When I was talking to Matt Burkett
about it, he pointed something else out to me that I had missed...it went something like this: "Under recoil, the pistol is already rising up and coming backwards towards you and recoil managment is all about getting the pistol forward and down again to prepare for the next shot...so why would you want to put your finger up there on the trigger guard and actually work against yourself by pulling backwards on the pistol before you even pull the trigger? That's just dumb!"
After using that grip for 2 years, I gave up and moved my hand back to where it belonged and I put my index finger down below the trigger guard. I found that I had better control of the pistol, especially when shooting full-power .45 ACP ammo. You won't find many serious shooters who still use this grip. It may work for some people but there's a distinct reason that 99% of the IDPA and IPSC world won't use it.