The compressor is like an engine, it always turns one direction. So the transmission has to decide what direction the car will move.....with that said liquid only goes one direction threw the compressor and the transfer valve is like the transmission if it goes into ac the valve sends the freon to the evaporator first then to the condenser. When it goes into heat mode the transfer valve switches into reverse and sends the freon to the condenser first then to the evaporator. The transfer valve , or reversing valve, is what makes it change directions
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