Bill, The jiffy sling is just your carrying sling set snug under your left upper arm (if you're right handed). It will help control the rifle if you're swinging for a moving shot. Hi-power competitors can't use a sling for the standing offhand stage, so they try to support the forend with their knuckles or fingertips and thumb, trying to get only bone from the forend down to the ground via forearm, ribcage, hip, leg, etc. they also use a shooting glove, or mitt and heavy, stiff shooting coat. Not practical for hunting, but the principle applies if shooting a stationary target. Practice position, aiming, dry-firing, and calling the "shot" when the trigger breaks. Shooting at bullseye targets helps develop good trigger control.... if your hit is outside your call, bad trigger control. If it's on, or inside call, good trigger control. If you start doing this, you will probably have what's called a 7 or 8 ring wobble. With a lot of practice. you can get it down to a 9 ring wobble. Few people ever consistently get to a 10 ring wobble, which in my opinion, is finally good enough to shoot at game offhand.
I am in general a lousy offhand shooter, and get panicky if I don't have something to rest on or against shooting at game. It is way different than shooting at targets.
There are shooters who seem to instinctively hit still or moving targets shooting offhand, but not many.
Offhand shooting is a good skill to have, but very hard to develop.
Good Hunting, Tom