Shooting off hand is the position that generates the least amount of felt recoil to the shooter, and standing with your left foot forward (right handed shooter) with a slight bend in your knee and leaning slightly forward into the gun, allows the recoil to move across your chest and down your rear leg, reducing the felt recoil that much more. Done properly the recoil comes straight back with a minimum of muzzle flip.
When the gun fires, your aim is disturbed by the recoil, but you follow through without taking cheek off the stock and regain your sight picture as quickly as possible. So the faster the gun cycles, the faster you can reacquire your sight picture, and it cycles fastest if the recoil assists the ejection of the spent round. That's why it is proper to have rearward pressure on the forend. In this way the spent cartridge seemingly ejects by itself, your push the slide forward as you lean forward, back into your shooting position, then its simply a matter of rolling onto your subsequent target and firing again as long as you have ammo and targets.
Edited to add . . .
If you dry fire a pump shotgun, at least a Mossberg, Remington, or Winchester, you'll note that it you exert rearward pressure on the forend, the slide won't come back when the trigger breaks until it nudges slightly forward. Thus you can never take the gun out of battery while there is gas pressure in the barrel. Recoil happens the moment the slug/shot column clears the muzzle, and the recoil provides the nudge which unlocks the slide, thus exerting rearward pressure on the forend is a benefit as it speeds the cycling of the gun, rather than waiting to recover from the recoil, then using muscle energy to cycle the gun.
the forend slides.
