The one where your watch warns you if your heart rate elevates above x BPM when you are idle. 120 BPM in my case. But don’t worry ... it’s because I had half a cocktail at a restaurant and got the “Asian flush” reaction. I really can’t handle alcohol. Frankly tho it’s amazing this feature works! I had a weird way of testing it, but I’m sure this feature will save actual lives! I got a warm fuzzy feeling seeing it actually work for me. Or maybe that was my drink.