That piece eliminates the rear shackles for the leaf springs. Its welded to the frame where the shackle pivot normally is. In a stock car there would most likely be no advantage. But in mine when im sliding sideways it will help the rear axle travel up and down adequately and locate the axle laterally much better than shackles. Oh ya and it will give the rear of the truck 2" of drop.
A pic is worth a thousand words.