AirPods in. Phone on the tripod behind you (rear camera), turned landscape. Tap start, then Calibrate β hit ~8 forehands and it learns your contact spacing. After that each forehand plays a low tone if you're too close, a high tone if you're too far, and the hit sound when you're just right. Nudge the band in/out and tighten it to shape where you make contact.
The trainer reads forehands best with the phone on its side β turn it landscape to keep scoring. (Scoring pauses while it's upright.)