How do I measure Heart Rate Variability? Intermittently, and with no pattern I can discern, my Watch measures "Heart Rate Variability". But I don't see a way to measure this on demand, and I can't find any apps that measure it directly (those that do use the camera and not Watch, and don't share with Apple Health).

How can I preform a HRV measurement on demand with Watch?
Looking at the history of HRV measurements in my Health app, they seem to correspond to times I've used the Breathe app on my phone. 

It seems that doing a Breathe session will do an on-demand measurement of HRV, and this agrees with a post in the forums on MacRumors.