Pronation is one of those tennis words that gets thrown around like everyone secretly agrees on what it means. Here's the plain version: pronation is the rotation of your forearm that snaps the racquet face through the ball at the last instant — the difference between a shove and a crack. It's real, it matters enormously, and it's also the one part of the serve that no camera can put a number on. Both of those facts are worth understanding.
What pronation is and feels like
Do this: hold your arm out in front of you, palm up, then rotate your forearm so the palm turns down. That turn is pronation. On a serve, it happens right at contact — the racquet approaches edge-first, and then your forearm rolls the face square through the ball and keeps rotating past it. That last-instant snap is where a huge amount of racquet-head speed and spin comes from, and it's why a relaxed, pronating arm produces so much more than a stiff one that shoves the strings at the ball the whole way.
You can feel the difference. A pushed serve keeps the strings facing the ball from start to finish — no rotation, no snap. A thrown serve leads with the edge and rolls through, and the follow-through naturally finishes with the racquet having rotated over. When people say a serve "cracks," pronation is a big part of what they're hearing.
Why no number can score it
Now the honesty, because it shapes what your feedback will and won't say. CoachVision tracks 17 body points and nothing in your hand — no racquet, and no fine forearm-rotation keypoint. That means pronation is not scored. There is no pronation number, and any tool that hands you a "pronation rating" from a phone video is inventing it.
What CoachVision does instead is honest: it reads pronation qualitatively from the contact and follow-through frames, the way a coach glancing at video would. From those frames it can note whether the strings looked like they faced the net the whole way (which reads as a push) or rolled through with the edge leading (which reads as a snap) — always hedged, never as a decimal. Pronation lives between the frames, in a rotation too fast and too fine for single-camera tracking to measure. So trust the feel of it, and treat the frame read as a coach's eye, not a stopwatch.
The drill — Edge-First Pronation Serves
You teach pronation by exaggerating the edge-first approach. Serve leading with the racquet's edge toward the ball, and let your forearm rotate the face through only at the very last instant. If the strings face the net the whole swing, you're still pushing; if the edge leads and the face snaps square right at contact, pronation has arrived.
Fifteen serves at half pace — slow, because you're rewiring a feel, not powering through it. Feel the edge lead and the forearm roll through at the end. As the snap becomes natural, the strings stop shoving and start cracking — and while no number will ever grade it for you, your ears, the contact frame, and the ball will all tell you it showed up.