I'm not seeing how that is real based soley off the video. With his hands behind his back he could have done the 'knap' which is the sound of a slap or it could have been added by the production crew. Also smith's hands barely seem to touch Rock's face and the exaggerated wind up and everything about the interaction reeks of stage combat.
E1: the way he pulls his off hand up to arrest his momentum is another way to reach this conclusion.
E2: his right foot is on his toes, his left foot lifts up onto his toes while he is swining the heel falls to late to be the "knap" but being off balance is another way to absorb the blow so the only thing moving is the arm.
Chris Rock’s face jiggles when smacked. View it on full screen.
He gets touched I won't contest that. However, if Will Smith really wanted to smack someone who was standing still with their face at shoulder height they would likely end up on the floor. At least he'd land the full hand instead of just tapping someone's face with his fingers.
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