Crossword clue: Waver, like watching a tennis match

The answer "YOYO" fits the clue because it describes the up-and-down motion of a yo-yo toy. When watching a tennis match, fans often move their heads back and forth, following the ball as it goes from one side of the court to the other. This side-to-side movement is similar to how a yo-yo moves. Both actions involve a kind of wavering or oscillation.