Typically, when we talk about the Apple Watch saving a life, 

 we are referring to the SP02 monitor, the ECG capability

the heart rate and blood oxygenation monitoring capabilities.

But according to Reddit user "xanderpy" (via AppleInsider), 

fall detection on the Apple Watch is what ultimately saved his mother's life. When an Apple Watch

user has a serious fall, this feature detects it, and if the user doesn't respond, 

 The Apple Watch's fall detection technology prevented a woman from dying.