Apple’s support site has a list of 12 features that watchOS 9’s new power saving mode points to.
Always turn off
▪️ Always in sight
▪️ Heart rate notifications for arrhythmias, high and low heart rates
▪️ Heart rate measurements in the background
▪️ Blood oxygen measurements in the background
▪️ First workout reminders
Turn off when iPhone is not around
▪️ Wi-Fi and Cellular
▪️ Incoming phone calls and maintenance
Do not shut down, but change the work
▪️ Making a phone call may take longer
▪️ Background app updates are common
▪️ Watch face data is updated less frequently
▪️ Siri can support request processing
▪️ Some animations and scrolling look less smooth
Function Fall detection We continue to work in power saving mode for security reasons. [Apple]
Source: Iphones RU
