I never sit on the clutch..... or the brakes for that matter so I have no idea how I keep warping front discs!!!
Anyway - I know that in the Supra the duration is about 2.0ms at idle, and drops to 1.5ms on overrun - therefore saving approximately 25% fuel. Like you say the stoichiometric AFR is (attempted to be) maintained at all times, but on overrun there is vaccum in the inlet manifold (technically that's not true - but less air than during running - hence engine braking). Just thought someone might know a bit about the ECU control and how clever it is.