Lend me your brains

So I guess my point is: you're going along as usual & the point at which you need to break grip is the same and has to happen before the lower gearing can have an effect.

The 'reverse' gearing takes effect as soon as you apply back-pressure on the pedals, or the rotation if the pedals (caused by the reverse gearing) catches up to your pedalling motion.
You would not need to start skidding, and could stop smoothly by resisting the rotation of the pedals.
More thought experiments! ;-)