What's the physics behind it?
there isn't any.
All the pretty videos and flow patterns show that this is not a recovery from VRS, it is simply using power to fly away from the incipient and very early stage, long before full VRS has developed. And you are right, the disc doesn't care which way the clean air is coming from - in fact if you have any forward speed then moving forward is the quickest way to reach undisturbed air.