Saves power by reducing current after initial pull-in. Pease recommended 0.8Rsolenoid for the series resistor, this uses 0.7Rsol. The time constant needs to be much longer than pull-in time to assure hard switching so the capacitor needs to be sized accordingly. Relay here is a typical Potter & Brumfield KUP-14D15-24. See the article about the circuit, "What's All This Solenoid Driver Stuff, Anyway?" by Paul Rako at electronicdesign.com.
