Double-J swatted the tennis balls, sending each in a wide arch and upon their return entagling their connecting strings. “These things are useless,” he said, with emphasis on the last word. “They can’t fight back. You don’t need to worry about making mistakes, because there’s no consequence if you fail. They lull you into a false sense of accomplishment. They don’t teach you any skill that will be useful in a bout. This isn’t practice — it’s a game.”