[He then leans his elbows on his knees, and just observes as to how her fists come in contact with the material. He can't recall the last time he was just able to watch someone else move -- his own trainers, maybe; or a glance at himself in the mirror. The other form of "watching" he was used to was, "watch for danger."
While he can admire the steadiness and regularity of her strikes, there is something about it that comes off as mechanic that makes him slightly uneasy -- to the point that he pauses mid-sip and barely realized that he was just doing what he did not intend to do: stare.
But if she wasn't missing a beat, like that, hey! Can you blame him?]