I've been told since I was a small kid,,,
To grip the rifle sling with your off hand,,,
Using it to pull the rifle snug into your shoulder.
When you shoot from a bench, or bipod, you dont use the bench or bipod to pull or push the rifle to your shoulder. You use your shooting hand.
Same thing in Off Hand. You non shooting hand is like the bench or bipod. The rifle rest on the hand.
When I mentioned not using a hard grip with the shooting hand on the pistol grip, I did memtion "firm Grip". Its this grip that pulls the rifle to the shoulder, but in doing so you dont have a hard grip which will interfere with trigger control.
To see what I mean, take a salt and pepper shaker. Draw a line on the table. Grip one of the shakers. Use your trigger finger to draw the second shaker down the like.
Grip the first shaker hard, and see how it disturbs the second shaker comeing straight down the line. Now loosen the grip a bit, you'll see its easier to keep the other shaker on the line.
You can still pull back the first shaker without a tight grip. The rifle is the same way.
Another test is to dry fire with a laser. (I'm a firm believer in laser sights for dry firing and training, both rifle and pistol). Steal your kids laser pointer (the one he uses to tormet the cat). Tape it to the from of the barrel.
Now as with the salt shakers, dry fire using a tight grip and watch what it does to the dot. Relax the grip and dry fire again. You'll see the difference. The harder you grip, the more it causes the trigger finger to move other then straight back.
Also pull the rifle into your shoulder without a hard grip, you can do it and still not interfer with the trigger control.