This unregulated (open-loop) circuit is a basic buck regulator whose output voltage is set by the duty cycle so that V_out = DC*V_in. The top NMOS connects the input to to the load resistor and output bulk decoupling capacitor when the PWM signal is high. The other NMOS turns on when the PWM signal is low and allows the inductor current to flow through the load and return through ground as its magnetic field collapses. The output bulk decoupling capacitor smooths V_out and reduces ripple. The switch on the output is to provide a simulation starting V_out rather than charging from 0 V every time.
