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It's my understanding that nowadays most heat pumps have a defrost cycle where they automatically run in reverse for a bit with some or all fans off

I think that may be how it works, yeah.

When it’s running in reverse then it’s acting as an air conditioner blowing cold air into the house. So usually the heat strips are used then to reheat the air and prevent it from blowing cold air in the middle of winter. Not strictly necessary but most people demand it.

auxiliary heat is a separate heat source for the indoors.

Most cold climate heat pumps run a defrost cycle to melt ice off the outdoor unit. that's different from auxiliary heat.

I see!

the aux heat comes in because their output is a multiplier. At 30F, perhaps they produce 4x the heat as the electricity put in. At 0F, perhaps they produce 1.8x the heat. This means the output declines with temperature, until eventually they don't produce enough heat to hold temperature. Enter aux heat.

Cold weather heat pumps help because they stay above 1x for longer, but you also wind up needing to oversize a bit.

Thank you for explaining!