The two transistors together form a PUT. The emitter of the PNP transistor I call terminal #A, the base of the PNP and the collector of the NPN I call terminal #B, and the emitter of the NPN I call terminal #C. When the voltage on terminal #A exceeds the voltage on terminal #B by about half a volt, the PUT starts conducting quickly dropping the voltage on the capacitor until the voltage between terminal #A and terminal #B drops below about 0.8V. Then, it stops conducting and the capacitor can charge again until the voltage on terminal #A once again exceeds the voltage on terminal #B by about half a volt.
