Cut and pasted from Chuck Hawkes. He's more articulate than I.
"Carrying straps are normally just a single length of material that attaches to the sling swivels at one end and loops through the second sling swivel, with some extra length and a buckle for adjustment purposes, at the other end. The true rifle sling is heavier and more complicated than a simple carrying strap, but it can also be used as an aid to shooting. Whelen (1 piece) and Military (2 piece) slings are both useful as shooting aids, although the Whelen type is easier to get into. They are both essentially double straps with claw hooks and keepers for adjustment purposes.
To help steady the rifle, both Whelen and Military slings are used with the lower part of the sling made into a loop that is pulled tight above the biceps of the left arm (of a right handed shooter). The left hand is then shoved tight against the sling swivel at the front of the forearm. When properly adjusted the sling should be tight when the left hand is in place against the forward sling swivel and the rifle butt is pressed tightly against the shoulder.
The "hasty" sling serves a similar purpose, but less efficiently. Instead of making the lower portion of the sling into a loop that is tightened above the biceps, the left arm (of a right hand shooter) is simply snaked between the sling and the rifle from left to right until the sling is across or above the biceps. Then the hand and arm is brought back under the sling to the left, and then up and slipped between the sling and the forearm from left to right and forced against the front swivel. When the sling is properly adjusted for length, it becomes tight when the butt of the rifle is forced against the shoulder and the left hand is forced against the forward sling swivel. Some carrying straps can also be adjusted for use as a hasty sling."
Full article is at
http://www.chuckhawks.com/sling.htm