This only benefits turbo cars. When the air is being PUSHED into the plenum, it's very turbulent and doesn't like to make the turn into the runners because of the turbulence. The dual plenum intake straightens out the flow into the runners by making it all go through that slot.
ITBs on turbo cars is pretty useless, since the real reason ITBs make more power is better, more equal intake characteristics. All the ITBs you see people put on cars have the little individual foam filters, but most race ITBs have plenums, and intake length has nothing to do with where the throttle is. A good intake manifold design can make an NA make power just as well, otherwise you'd see lots more street cars with ITBs just so works teams could run them on their race cars. The last time ITBs were put on a street car like one we could afford (ie, not the super cars) was the AE111 Corolla on the 20V 4AGE in the 90s.