Some releases are made for loops and some caliper/jaw styles have jaws that are to wide to easily hook to a release. I wouldn't shoot without a loop any longer, I use a scott rhino release, its designed for a loop and shoots with the index finger, no jaws to close, just a post to hook inside the loop and ready to shoot.

Whoever said not to use a thumb release with a loop is dead wrong, thumb releases, such as the Carter release line, are EXCELLENT for loops. However BT releases with no trigger need adjusting to be set right for a loop, has to do with the angle of the sear before firing.

Most thumb releases can be shot with/without a loop, when shooting with a loop, they normally have a piece of loop material on the release which you wrap around the string, back to the release gate. This in effect is a loop, its just on the release, not the string.

My wife hunts with a Carter Insatiable release (thumb style) and a loop, once in the tree she hooks the release onto the loop and there it hangs til shooting time.