In this circuit, the low pressure switch (S2) makes first, supplying switched 12V to the relay (RY2) coil, no lock up at this point. As pressure continues to rise the high pressure switch (S1) makes and supplies ground to the LU solenoid and the relay (RY2) coil, energizing the relay (RY2) and the L/U solenoid. As pressure drops the high pressure switch (S1) opens, but the relay is wired to self latch on the ground side, this is the jumper from the output of the relay contacts back to the relay coil -. The low pressure switch is still energized at this point keeping switched 12V on the relay coil + and the relay stays closed and L/U engaged. Once pressure drops off enough to open the low pressure switch it removes the switched 12V from the relay coil + and the relay opens, this will also open the ground latch. L/U is now de-energised. Once pressure starts rising the cycle starts again.

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