The circuit is based around the standard 555 astable circuit, with a couple of alterations, namely the wiper of VR1 is connected to pin 7 and the addition of D1. This configuration means that the frequency can stay constant but the mark/space ration (on/off time) can be altered quite dramatically, from about 1% to 99% thus making an ideal PWM voltage controller.
Q1 is a Darlington PNP type with up to 5A of current drive, however you must use a large heatsink which will get hot, and do not exceed 65 Watts. This can be replaced with a higher rated device such as TIP147 for up to 125W!! This is enough to operate PCB drills!!