Let's begin with -12 volts on the NEG PWR IN line and zero volts on the NEG TRIGGER line. The relay 12 volt coil K1 is unenergized and the relay contact K1A is open.
Apply -12 volts to the NEG TRIGGER LINE just long enough to pull in the relay contact. Coil current flows from the NEG TRIGGER LINE through D1 and energizes K1. The relay contact closes, applying -12 volts to D2 and supplying approximately -11 volts to K1. The relay is latched in the ON state.