If you are used to simple, straight cool split systems you know that the low voltage to the outdoor unit is usually VERY simple with just a Y (contactor power) and a C (common) connected to the outdoor unit in many cases. When the condensing unit controls are strictly two-wire low voltage there is no continuous low voltage power so there are also no timers or other logic in the condensing unit. Usually, in these cases, the LV wires connect directly to the contactor coil.
A heat pump needs to be able to switch between heat and cool and defrost which brings in the necessity for more control conductors and complexity.
A heat pump defrost board like most modern controls contain both loads and switches to control different functions. because it has timers and some basic “logic” the board requires a power supply and for most residential split system boards this power comes from the C (common) and R (hot) terminals from the indoor 24v transformer.
The defrost board also utilizes the constant power on the defrost board R terminal to back feed voltage through the W2 wire back to the secondary heat inside whether it be heat strips, furnace or hydronic secondary heat.
This helps to counteract the cooling effect that occurs when the heat pump when it shifts from heat to cool mode for defrosting. This function is an important thing to test on heat pumps to reduce cold draft complaints during the winter.
Simply force the board into a defrost and check for 24v between w2 and c at the outside board to confirm proper operation or check the secondary heat via ammeter or visual confirmation during the defrost cycle.