What makes these slings unusual is that there are four different lengths: Short, Medium, Long, and Extra Long. Both grips on bows and hands are all different sizes, so getting the correct length can make a dramatic difference in in both technique and performance. Additionally, they are handy when using stretch tubing for training, as the tubing leaves the hand when shot. The sling keeps it from flying some distance.
Choosing the correct length – Depending on the size of the grip area of the bow and the size of the hand, different people might need different length slings.
For an average recurve bow grip and average size hand, the Long is generally better. It allows the bow to leave the hand properly without either keeping it in the hand (too short) or allowing the bow to flop around at the very end of the shot (too long). Smaller hands need a longer sling, as the sling must still go around the standard grip; a larger hand maybe can do with one a size shorter.
For almost all newer compound bows, the Short is best because the grip area of the bow is so small where the hand sits and is quite small in circumference. Old compounds have grips more like a recurve, so a Long or even an Extra Long is recommended. Some of those grips are even larger than a recurve grip.