It can work in two ways, first if you want to maintain a certain pressure in a vessel(tank), the cut out pressure should be set for that pressure I.E 120psi so that it cuts power to the pump when that pressre is reached and to maintain enough pressure in the tank so your equipment runs properly, you set the cut in pressure to about 15 psi less so the pump comes on to restore the 120 psi you need. Second, if used in a boiler or furnace, it is simply a on/off switch. When the exhaust motor starts, it creates either a positive or negative pressure in the switch allowing the switch to make or break power to continue or discontinue the sequence of operation.
