Though Benedict may be slightly concerned by the shift in Atticus' demeanor, he has no reason to think anyone would ever have cause-- or the gall-- to strike him. For this reason he doesn't even think to cringe before it happens, and the blow hits him full force with a strangled yelp, jolting him to the end of his own chain. It's too short for him to go far, but he gets a nice abrasion on his wrist from the shackle, and he claps his free hand to the struck side of his face, looking at Atticus in horror.