Was talking to my mechanic about this. Apparently killing the throttle isn't the first thing the BMW system does. The first stage is to apply the rear brakes alternately at the left and right wheel. This happens without reducing power - and it's often enough to get the car going from stand-still. Cutting the power is the second stage, and it only lasts briefly before we're back to the first stage of braking.
Again I'll note my caveats - I haven't actually been stuck yet - so I don't know about that. And my wife's 335 is an auto - so I don't know if that means the TC system is any different to a manual car. None of my manual bimmers have traction control .. so many variables!