Just purchased the Watch 7, long time member of the ecosystem. I think it would be a great addition to the software to let the watch account for having the face placed on the underside of the wrist - most of the time it functions just fine, but it's not as intended and some gestures don't work properly.
Wearing it on the underside of your wrist confuses the sensors as well. The sensors were made to work on the top side of the wrist, the way most people wear their watches, so any health readings YOU get may not be accurate. If you don't care about the health features, one wonders why you went for the top of the line. As you can already see, the gestures don't work properly. You may as well turn them off. If they ever do get the BP monitor working, it most certainly will not work on the underside of the wrist.
