Can someone clarify how the GHL operates with a bottom heater?

According to the manual the bottom heater takes precedence over the heater. The heater is switched on only when the bottom heating alone is inadequate.

This feature is potentially useful to me, as I have a heatpump that is the prefered heating method, with titanium heating elements as a backup. So I could set the heatpump as the "bottom heater" and the titanium heaters as the "heater".

But I am unclear exactly when/how the GHL determines the bottom heater can't cope and kicks in the main heater. Is there a built in time lag? Or does it wait for the temperature to drop even further? What is the logic process the GHL follows before turning on the main heater?

Thanks,

Keith