Heart rate sensor
why does the heart rate sensor work when the apple watch isn’t on my wrist ? It shouldn’t work cause it gives inaccurate data
Apple Watch Series 3, watchOS 4.2.3
Hello,
For best results
Start with a good fit. Even under ideal conditions, Apple Watch may not be able to get a reliable heart rate reading every time for everybody. And for a small percentage of users, various factors may make it impossible to get any heart rate reading at all. But there are things you can do to help Apple Watch get the most consistent and best heart rate readings possible. Learn what else affects your reading.
From:
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support