Where can I see that? Surely you're not referring to that Reddit video? I've seen that one, but my interpretation is that Yiheng was somehow unruly, she held him back (by his arm) and I guess told him to stop it, but he didn't listen so he got a little slap and she pointed to the floor, I guess accompanied by telling him to sit down. Nowhere in that video (or in the Reddit thread) did I see any evidence that she did that because he wasn't solving fast.