With a traditional, mechanical throttle linkage, there's a momentary spike in emissions caused by a too-rich or too-lean combustion condition when you suddenly lift off or step on the throttle because the engine computer and fuel injection system can't compensate quickly enough. Throttle-by-wire allows the manufacturers to smooth the throttle position changes. Even when you snap your foot off the pedal, the throttle valve is now made to close over some significant fraction of a second, slowly and gradually enough for the fuel mixture control to keep pace.