It basically reduces the turbulence that happens from the front pressurised turbo air that meets at right angles to the pressurised air from the rear turbo.
Putting that splitter in forces the air to take the same direction as the air from the rear turbo. Think of two water hoses fed into a big hollow pipe, if you shoot one down the end of the pipe and stick the other hose in at right angles you upset the flow of water out the end. Now if you direct both hoses in the same direction, you will get a less turbulent, 'cleaner' flow down the pipe.
Nissan thought meh, for a standard car, but Nismo fixed it with their turbo outlet piping kit, similar to the HKS setup.
So instead of the boost fluctuating at light to medium throttle loads etc, it is a lot more stable on boost, and car drives nicer!
Would even help a stock turboed GTR up GT-RS (-10) turbo equipped 32-34GTRS (okay, maybe not -10's, GT-RS's)