After watching the slo-mo version, it looks fantastically staged. Rock has his neck craned out to set up the slap and Will is a good physical actor for a staged slap...barely phased him and barely made him move, physically. As stated above, Rock would've had a much harsher reaction if it was real, due to the size disparity between the 2. If you look at Rock's face just after the slap, he glances at the audience for the reaction with a shart-eating grin...even softens the blow, himself, by moving with the slap before he's hit with it.

HERE