Board Thread:General Discussion/@comment-29936979-20171127042934/@comment-33674287-20171127225958

Every character does something like that.

For example, RG01 and -02 bob up and down while swinging their swords, Froggit moves its head from side to side, and Undyne does something similar to Sans, bobbing up and down. I think it's meant to simulate breathing, as Sans's movements slow near the end of the battle (as he gets tired).