This article is from the Fencing FAQ, by Morgan Burke with numerous contributions by others.
These are modern, orthopedic grips, shaped vaguely like a pistol,
but still grasped in the traditional way. They provide a
pronounced strength advantage over the traditional grips, but can
encourage wrist movement over finger movement. Pistol grips all
have the features of a large protuberance below the tang for the
aids to grasp, a curved prong above the tang that fits in the
crook of the thumb, and a large prong that extends along the
inside of the wrist. There are many variations in shape, size,
sculpting for the fingers, extra prongs, and so on, although
certain designs enjoy wide popularity. Most pistol grip designs
have names (eg. Visconti, Belgian, German, etc.) but these are
not always consistent between manufacturers or regions.