It is used to set up a two point sling. For target shooting, the sling was about 2" wide, although a standard sling can be rigged this way as well.
With the sling hanging down, give it a half twist anticlockwise, looking down from the top. Put your left arm through, positionning the sling high around the upper arm. Bring the left hand aroung, so tyhat the sling lies across the back of the hand, and grip the forestock. When the length is properly adjusted, the rifle will be well supported, and also stabilized laterally.