If the aux heat is a gas furnace inside that you want to kick on instead of the heat pump then you need to set a temperature that Nest will shut off the outside unit and switch to the other heating system (a gas furnace for example) :
If you have a dual fuel system, a type of system with a heat pump outside and a separate heater installed inside your home, you won't be able to use Heat Pump Balance. If you have a dual fuel system, you can set a lockout temperature manually in the Nest thermostat's Equipment menu.
Heat Pump Balance options
Off: If Heat Pump Balance is off, your thermostat will use your AUX heat based on a lockout temperature that you set. The AUX lockout temperature can be changed in