It’s a practice probably as old as the game itself: coaching from the stands. It comes in many forms: subtle nods, a shifting of a hat, a wink or sometimes out-and-out verbal instructions. The procedure itself is illegal. The enforcement of said legality is a gray area that gains more scrutiny by the year.
The accusations have dogged Rafael Nadal since he arrived on the worldwide tennis scene, if you even want to call them accusations. If you watch his Uncle Toni during a match, you can see him and Nadal often make eye contact and I doubt it’s to bird dog Kate Middleton when she’s in attendance.