basically the single piston valve stays slightly open on idle, this leaks air that has been metered by the airflow meter, thus telling the ecu there is more air coming into the engine than actually is (coz some is being leaked via the valve) so the ECU fuels for the meter amount, thus over fueling the amount of air it actually gets causing bad lumpy stalling idle.
twin piston was designed for air flow metered cars, i.e S12, but mainly the RS-turbo, now they are a standard valve coz of reliability.
i think thats the best explaination i can give as what i was told when i got mine.
P.s you may have been better to look towards Ebay for parts like this, you can pick up bits much cheaper, only bad point is the lack of after sales problem solving.