Use a knuckle to support the firearm not the back of the hand. (Using the back of the hand causes us to expose more of the head from cover. This also causes the weapon to recoil more aggressively.
I'm trying to visualize this...when shooting from behind a barricade my stance and two handed grip are exactly the same (I use a slightly modified isosceles, thumbs forward grip), I am tilting my upper body and leaning as need be to see the targets. I am usually at least an arm's length from the barricade.
From your quote I'm "seeing" an aggressive forward lean, gun in one hand (preferably strong hand???), support hand resting on the barricade (or cover or concealment) with the hand parallel to the ground using the middle knuckle of the middle or index finger as a resting place for the gun. If that's not right can you paint a clearer picture of that statement???