I've seen them... I think the concept basically lies in the fact that it's a shell of a grip that "free-floats" over the riser or something like that? Ostensibly, it allows you to grip the handle as hard as you want without imparting torque to the bow itself, I sould surmise.

Here's my take on it: It's a LOT easier and WAY more beneficial just to learn how to grip a bow properly in the first place.