Here is something pulled off of another drumline forum:
The Garfield Grip is the norm for cymbal grips. Its the one where you slide your whole hand through the straps and then turn your wrist over so the that strap lies between your index and your thumb. It lets you use your whole hand to control the cymbal. The pistol grip was mainly used, and made popular(at least as far as I know) by the Scouts. Supposedly it was used because it made the flipping the cymbals and visuals easier. Not sure if that is correct but it's what I was told. In my opinion, unless wad-burners are being done extensively, the Garfield is the way to go. Especially with a young line.
With the Pistol Grip, the straps have to be much tighter than when using the Garfield Grip. Most straps will probably require a good amount of cutting in order to get them tight enough. Anyways, with the Pistol Grip, you only put 4 fingers in the strap as opposed to using your whole hand with the Garfield. The strap will rest between your index finger and your thumb. Once your finges are in its easy, you just make your hand look like a pistol. Your index finger will be pointing out, your thumb will make a 90 degree angle with your index and the other fingers will be tucked between the strap and the cymbal. I can't remember if the Scouts had only there index finger out or if they used both the index and the middle finger. I know there are some old Scouts guys on here, maybe the can chime in.
VK 1994, Pit
Blue Devils 1998, Pit