What does the emergency heat do on a heat pump?

Brings on back up heat, either electric "resistive" heat or a gas furnace, for when the heat pump can't keep up any more due to cold outside air temps.

Yes we have a great heat pump here in Tennessee, it can cool or warm air a certain amount. So on a real cold cold day outside, the heat pump can not warm it up to a level we may want. ( or we merely put on warmer clothes) But we can turn on the back up heat ( or emergancy heat button) and on ours heating coils that are electric turn on to warm the air more. ( and of course you watch your electric meter go wild as it does)

It shuts off the heat pump and heats with the electric heat inside the blower unit. Use it if the heat pump can't keep up ( temps below 20 ) or if the out side unit quits working.