Leftmost potentiometer substitutes water. When resistance goes very high the Schmitt Trigger turns on driving the power transistor at the rightmost part and current flows to the 13 ohm motor load.
The motor continues to turn on until water level increases sufficiently to lessen the resistance and turn Schmitt Trigger low.
The low output will not reach the breakdown voltage of the zener diode thus almost no current flows to the motor.
