Well.......glad ya asked!
And I quote: (paraphrasing in places)
More than just a fancy add-on, ... the girdles are made from strengthened aluminum and feature
thick flanges, structural bracing, and fill and drain plugs to simplify fluid servicing. These girdles are also a bit larger than standard covers because it helps them clear larger ring gears and
allows for an extra half quart of rear fluid. Yet
the real benefit to any girdle lies in the two load bolts that are integrated into each design. The load bolts can be seen protruding out the back of the covers; I'll explain their purpose in a moment.
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During a drag race launch, the pinion gear is smacked into action, and it slams the ring gear and sends it into motion. At the same time, the torque delivered by the engine, transmission, and driveshaft drives the pinion, ring gear, and differential rearward. In simple terms, the force tries to smash the differential out the back of the rear housing.
The loading is only resisted by the main caps, which hold the differential in place. Often, the force is severe enough that it distorts the main caps and causes them to crack and fail. The loading also causes the ring-and-pinion gears to spread apart from each other, leaving the skinny ends of the teeth in contact, the result of which are broken gears.
The good news it that aftermarket girdles are stronger than stock stamped-steel covers found on most factory rears, and, as I mentioned above, they benefit from the loading bolts that are fitted through the covers. Each of the two loading bolts has a swivel foot on the end that butts up against the center of the main caps and preloads the cap to counteract the loading seen on launch or hard acceleration. Though the system sounds simple, it has a wonderful effect, often increasing the longevity of internal parts.
