To better understand a "Diverter Valve" think of a Y-pipe. In the Y there is a valve. Normally you want the flow of air to go to the LEFT. Under normall circumstances it does. BUT... when the boost levels go up, inside the vavle is a spring loaded diaphram. As boost levels build... they press against the valve's diaphram and bleed off air. Also under high boost levels the valve is slow to close fully.
When you lift off of the gas like to change a gear... the divirter valve will open and the air flow will 'dirvert" off to the RIGHT of the Y valve. When the valve closes it has to work against the flow of air pressure (boost level).
Now... with an aftermarket diverter valve... the inner spring is stronger. So... it does not bleed off any air pressure (ie boost) and when it opens and closes it is faster than a stock valve. This is where the gains are. None to little bleed off and better cycling of teh valve from open to closed positions.
Hope this helps you better understand the diverter valves.
bye,
David