Best accuracy: film with your phone's slow-motion camera (120–240 fps), phone still, ball and the first 1–2 m of flight in view.
Side view (recommended): phone on the ground or a tripod about 1.2–2 m in front of you (facing your chest), lens at ball height, landscape, pointing straight at the ball. The ball should sit in the left third of the picture if the target is to the right.
Down the line: phone 1.5–2.5 m behind the ball on the target line, lens low. Gives the start direction (left/right) better, speed less exact.
Open 📷 Swing, drag the circle over the ball, and set the camera-to-ball distance. Tap Detect ball. The ball's real size (42.67 mm) sets the scale, so no tape measure is needed for side view. The circle turns green and the app is armed.
Swing normally. When the ball leaves the circle the app saves the swing, tracks the ball in the first frames and shows a slow-motion replay with the track. Tap Fly it to send it down the course.
After each camera or video shot the app runs a body-pose model (offline, on the phone) over the swing and gives a form score, the top 3 things to work on with drills, a slow-motion skeleton replay, key positions, tempo and hand/arm speed. Your whole body must be in the picture (head to feet): put the phone 3–4 m away — for ball tracking at that distance pick 640 px tracking resolution, or record a video. Set your height in Settings for correct distances and speeds.
Face-on (phone in front of your chest) checks head movement, sway, weight shift, reverse pivot, shoulder/hip turn, lead arm, chicken wing and finish. Down the line (behind the hands, looking at the target) checks posture, spine angle, early extension, knee flex, over-the-top, lead arm and finish. Only the checks that a given view can see are shown.
All coach numbers are estimates from a single camera: depth is guessed, landmarks jitter and fast motion blurs at 30 fps. Slow-motion video (120–240 fps) gives much better speed and timing numbers. Treat them as a guide and watch the trends.