It gives an index point and room to get the support hand fingers a little bit higher in relation to the bore. Seems small, and seems like it wouldn't do much of anything, but small changes in tension and positioning have a HUGE effect when it comes to handguns. I noticed a definite performance improvement after doing it. It also has the effect of "locking" your support hand index finger in place. Here's the best pics I could find of the way I do the thumbs forward grip, from all the angles I could find. Notice the placement of the upper palm / base of the thumb, support hand index finger, etc.
Now notice the difference in support hand position in comparison to this grip. Support palm / thumb base is significantly farther down the grip panel (a solid 1-1.5" lower), support fingers are likely resting directly on top of gun hand fingers, instead of more in the "grooves" between the gun hand fingers. Ultimately this equals less control of recoil:
Finally got mine how I like it. G23 Gen 3 with some stippling and grinding done, Meprolight sights, LW 40 extended barrel, LW 9mm conversion barrel, 357 sig barrel and looking for a cheap 22lr conversion. This is my hunting set up. I know they say 40 is a little light for hunting, but I'll be in my bow blind with 20 yard shot or less and Underwood Ammo has some 155grn XTP advertised at 1300 fps with reviews show it hitting 1400 fps in an extended barrel. That should have plenty of thump for deer and pigs.
Had a bunch of 9mm ammo and needed something to shoot them, so yesterday I went and got a Gen4 G34. First Glock I have bought since the mid 90's. I do like the enhancements that the Gen4 has.
I got such a smoking deal. It was bought new and traded in a week later cause the guy said his hands were too small for it. I don't know if he even tried the back straps, they were all in the box. I got it for $575 so it was a must have.
I love shooting it. The sight radius is awesome. It had the (-) connector, but the trigger pull was still about 5lbs so I swapped it for a 3.5 Ghost.
I also have a G21C and a G30 so I have my share of 45ACP Glocks.