It's a fundamentally stupid idea, the sensors in the watch are woefully insufficient to detect such an event with any reasonable amount of precision. Bumping your watch into a tree would generate massive G forces, hitting your head while your arms are flailing would barely register.

At minimum, it would require biosensors that detect the owner is still wearing the watch yet his life signs are significantly perturbed from the norm: irregular heart beat and oxygen saturation, hypothermia, AI-informed sound interpretation for things like gasping for air etc.