The stock being tucked back behind the bicep shortens the overall length of what looks like a long shotgun for the task at hand.
When you find a threat, the shotgun is pushed forward as the muzzle is brought up. Buttstock slides neatly into the pocket of the shoulder. Works like a champ.
Viola.
Folks need to take more training from knowledgeable folks.
Referred to as "Short-stocking" the Scattergat, used in very close quarters where a conventional Buttstock makes the overall "Package" too long/unwieldy, IIRC--Not that I would know anything about such things...