The standard pipe causes a restriction in air flow because the air coming from the front turbo merges with the air from the rear turbo at almost 90° causing turbulence and a loss of efficiency. By placing a baffle between the two half's, the air from the front turbo can be "bent" so that it meets the rear turbo in the same direction.
The added benefits are that it can increase throttle response, help cut down on turbo shuffle and increase output by 10-15hp.
The modification is quite simple, just requires an aluminium plate to be welded inside the pipe, or you can buy a pre-modded version from the GTR Owners Club shop.
I will have a go at fitting my pipe at the weekend if it is dry.