Seek explanation of heart rate synching of Notify with Mi Fit app and Mi Band

Joggle

I'm trying to understanding how Notify interacts with the Mi Band and the Mi Fit app to display heart rate. It doesn't seem intuitive.

For example: I switch off all heart rate monitoring in the Mi Fit app and if I set the Notify app to "Use Notify app only". The Notify app still gets new data when I synch the Mi Fit app with the Mi Band. (If you see what I mean.)

How is that happening? The Mi Fit app shouldn't be getting any new data from a sync and, even if it does, the Notify should be ignoring it

If there's an explanation posted here previously about how the two apps and band work together, then could someone kindly point me to it. Thank you.