He punched Silva full-on in the face while Silva was clowning. The frame rate of that gif sucks. See the video and watch the punch connect:
http://www.youtube.com/watch?v=FcTHhnBDogs
Edited to add: The chin is the worst place to get hit, as it starts the head rotating, which leads to reduced brain function.
Still think that punch missed? See a closeup in slo-mo:
http://www.youtube.com/watch?v=Jh0DMWhDP-s