When I shoot off the bench, with both left and right elbow rests (no rear bag), I hold the forend as if holding the rifle offhand, and rest the back of my left hand on the sand bag. It is a very steady hold. This method gives me exactly the same point of impact as offhand shooting, or shooting sitting.
If I let the forend bounce on the bag, not held, the impact is different than when shot offhand- My modern rifles and the muzzleloaders all react this way.