Scythefwd;
I think you are talking about a real competition sling like is used in
smallbore and high power competition.
The hook is used so you can have your sling on your arm, adjusted correctly for the position you will be shooting (Prone, sitting and kneeling) and not have to remove the sling from your arm, just pop the hook loose.
A properly adjusted competition sling will have the loop around your upper arm, generally just above the bicep / tricep. (some like it a bit, higher, some like it a bit lower.) The keeper is kept down toward the hook until the sling is positioned (by your left hand) where you want it on you upper arm, then slid up (by your right hand) to snug the loop around your upper arm.
Correct adjustment is when you are in firing position, left hand under the forestock, sling essentially straight back from the hook, under your hand & then off to your upper arm. The sling should be snug, pulling the rifle back into your shoulder. It can be a little uncomfortable. Each rifleman will find quickly how snug they like their sling, a little trial and error will show you.
Once you have gotten the adjustment and feel comfortable, you scores will improve significantly for all positions used. (except offhand where the hasty sling is suggested)
If you use a shooting jacket, the sling usually stays on the jacket, held in position with a keeper of some sort. (strap, hook etc)
Have fun and good shooting.
Roger
