Hopefully someone can explain how this relay works?

Dana is on the right track. 79 C1 at the PCM is controlling the ground side of the relay which is a common method of switching. When the PCM pin "goes low" the relay triggers.

The relay top right terminal is fed from some main buss at it's 20A and branches off somewhere else. That is power to the relay coil

Power for the switched load is coming in at top left, the switched loads are supplied out the bottom left