Brass in the shoulder area is thinner than brass near the base, so adheres to chamber wall first as pressure develops. Base returns to bolt face as pressure develops, and a stretch occurs just above the belt. If there's enough stretch on first firing the case is ruined. Faux shoulder keeps shoulder tight up front, ergo base tight against bolt face, on firing pin impact. So there is no/less thinning above the belt.
Creation of a faux shoulder on virgin brass as I, Stick, and Smith describe is a very effective (altho not perfect) way to prevent ruining cases on first firing in generous chambers, thereby granting cases a near-normal lifespan. The bottom line here, regardless of how you parse mechanism, is that this approach works. As said, I don't do it on just any belted mag rifle - just on those with very generous chambers. It is a bit of a PIA.
