How can a heat pump warm a house by causing a refrigerant to evaporate and condense

The hot discharge gas from the refrigerant compressor is normally cooled and condensed at high pressure. This is then passed through an 'Expansion' valve which decreases the pressure to a low level causing expansion of the refrigerant liquid. 
The liquid partially vapourises causing a 'Joule's/Thompson' refrigeration effect' which decreases temperature of the refrigerant which then passes to an evaporator coil in the air circulation system of the building. 
In the evaporator coil, the heat exchange between the cold refrigerant and the warm air of the building, vaporises and heats the refrigerant which returns to the compressor. 
The cycle is repeated until the air temperature reaches the thermostat set-point and switches off the system. 

As a Heat pump, the hot refrigerant gas is not evaporating and condensing. 
From the compressor discharge, the hot gas is by-passing the cooler/condenser unit and the expansion valve and passes directly to the 'evaporator' coils but now, as the heating medium for the air circulation system where it's cooled by the heat exchange between the hot gas and the cooler air in the building and returns to the compressor in a continuous cycle. 

A Thermostat in the system starts and stops the compressor motor according to the heat or cool temperature settings.