In the 3's (and other mazdas/fords) when you have the vehicle in gear and let your foot off the gas (decelerating) - I believe it shuts the injectors down to just enough to barely keep the engine running. This provides engine braking and fuel savings when coming to a stop. It doesn't do this when it's in neutral though, as the car would die due to no load.
If you look at the ECU - it has a line-in for brake input (my FS did, my KL does). I believe this is what triggers that function.
If you look at the ECU - it has a line-in for brake input (my FS did, my KL does). I believe this is what triggers that function.