AFAIK, closed loop is when it uses the O2 sensors in the exhaust to judge the AFR and adjusts fueling to try and hit the target AFR. Open loop is when it doesnt use the sensors to adjust it, either because the cats arent up to temp (so wont get a proper reading from the O2 sensors) or at WOT, here I beleive it uses a lookup table to guess what fueling it should use. As I say, thats how I understand it, sure Abbey will correct me if I'm wrong!
Ahh ok Chris thanks. The ecu's must have to work pretty fast then.