When apple first released the low power mode for the new apple watch ultra it just reduced some background apps while keeping the workout app preserved. Which would keep the readings for heart rate and gps at 1 second. However, the new update for the lower power mode that will be coming "later this fall" has changed that. It will now have the reading for the heart rate at every minute and the gps reading every 2 minutes. In this low power mode, this allows up to 60 hours of battery life.