The legs keep running while the arms wave. A second animator on the same model is told which bones are its own with only, and goes on top of the first.
A bone comes with everything that hangs from it, so 'spine' is the chest, the head and both arms. When its movement ends it hands the bones back, eased by its fade, without anybody asking.
Click the preview first so it hears the keyboard. Space waves, 1 or 2 change what the legs are doing.