Short stroke piston actions "whack" the BCG, transferring the energy to move the BCG several inches in as little as the initial 1/4" of travel.
Think of a car at speed slamming into a stopped car with enough energy to move the stopped car.
The long stroke PWS action pushes the BCG through its travel. The same energy is transferred over a longer period so the inital impulse is not as harsh.
Admittedly even the worst recoil impulse from 5.56 NATO is not much but last week I fired 2 DI ARs, an AUG, a Hellion, and my current favorite, a PWS MK114 MOD 2-M upper with an SSA trigger and a Gen IIe Razor 1-6. The PWS equipped AR was by far the easiest for followup shots and had noticeably less recoil.