Pins 1, 2 are used to loop the ground connection from the 12V power input to pressure switch contacts, and back to the relay. When the pressure switch closes, and relay is energized, AC power switches from Pin 3 to PUMP 1 connected to X1 - 1.

The device, approximately 1.5x1.5 inches can be encapsulated with pigtails, or made in such a way it can be mounted on/in an electrical enclosure commonly used in construction.

A version with softstart may be useful, guaranteeing the relay cannot operate for a period of time, or until an input enable signal is received.

I could call it an "EXCLUSIVITY POWER SWITCH" or "NON-CONCURRENCY SWITCH." The switch shown is rated for 16 amps.