If a coach tells you they know "the secret to tennis," be skeptical. There is no secret. Tennis is genuinely hard, it takes years to develop, and anyone promising a shortcut is selling something.

But if we're talking about the single most important technical concept in the game — the one thing that separates recreational players from elite ones at a biomechanical level — there is an answer. And it's something most players never consciously understand, even after years of playing.

It's called the 70/30 rule.