Croûtage
skinning
Croûtage — from the French croûte, "crust" — is leaving piped macaron shells in the air until a thin dry film forms on the surface, usually 20 minutes to an hour.
That film directs the rise. It blocks steam from escaping upwards, so the expanding air moves sideways and forms the foot, while the dome stays smooth and does not crack. Without skinning, shells generally split across the top in the oven.
Readiness is checked with a fingertip: the surface should be matte and not tacky, and give slightly under light pressure without marking. Timing depends heavily on humidity — in damp weather the film may not form at all, and the room has to be dried or a fan brought in.
Leaving them too long is also a fault: too thick a skin restricts the rise, and the foot comes out low.