1) As light falls on the solar cell and the capacitor charges with respect to time and the voltage starts increasing across the capacitor.
2) Initially all the transistors are off (no current in the circuit) .
3) As both the transistors are off the only path remaining for the voltage to appear is through motor, 2.2K resistor and zener diode.
4) Zener diode used has a 2.7Vzener voltage, which means itwill conduct in reverse bias after crossing 2.7V at cathode.
6) As the voltage of capacitor reaches 2.7V i.e. the same voltage required to conduct zener in reverse, will start conducting.
7) As zener is ON ,the PNP (2N3906) transistor will get a base drive (signal) and will turn on.
8) Now the PNP transistor turns ON the NPN transistor because they are connected in positive feedback loop.
9) As both the transistors get into saturation region (ON state) the circuit completes and the motor starts running and capacitor starts discharging.
10) If you have noticed the main aim of the zener diode was to trigger at 2.7V and after that it hardly plays any role.
11) After triggering the circuit at 2.7V both the transistors are turned ON and they remain ON till 0.7V (minimum voltage required to conduct transistor). As both the transistors are connected in positive feedback loop the PNP will keep the NPN ON and the NPN will keep the PNP ON(Both the transistors are turned OFF below 0.7V ).
12) As soon as the voltage across capacitorgoes below 0.7V the circuit gets off, the motor stops and the capacitor starts charging again.