The FLED SE operation is relatively simple to explain.

1. The solar cell charges the storage capacitor from zero towards the 
trigger level during which time the PNP and, in turn, the NPN 
transistors remain off. 

2. The voltage across the storage cap is also connected across the 
series resistor and FLED but no current flows until the voltage  
across the FLED exceeds the minimum turn on voltage of about 2V.

3. Above 2V the FLED starts to operate dimly turning on the LED, with 
small short duration current pulses and low average current.

4. These short current pulses through the FLED cause small voltage 
pulses across the series resistor. 

5. When the voltage across the FLED is 2.5V, the current pulses are 
large enough to generate voltage pulses across the series resistor 
which exceed about 0.5V, the base emitter turn on voltage of the PNP 
transistor.

6. When current pulses flow through the PNP base emitter, the PNP 
collector starts to turn on and pass an amplified PNP collector 
current.

7. The PNP collector current flows through the NPN base-emitter 
junction and causes the NPN collector to pass even larger current 
pulses.

8. At the critical stage just before trigger, the NPN collector 
current pulses flowing through the motor winding, cause voltage 
pulses across the motor winding which pulses the NPN collector and  
normally positive side of the FLED series resistor towards ground 
potential.

9. The circuit achieves greater than unity positive feedback and 
triggers when the current pulses from the NPN collector flowing 
through the series resistor (and through the PNP base) are greater 
than the current pulses of the FLED.

10. When the SE triggers, the PNP/NPN pair supply sufficient base 
current to each other to latch on like a SCR, with the PNP base 
current supplied by the NPN collector and the NPN base current 
supplied by the PNP collector.             

11. The NPN turns fully on and the + side of the storage cap is 
discharged through the motor winding and the NPN collector to Gnd.

12. When the voltage on the storage cap drops below 0.6V, the PNP and 
NPN base emitter sharply drop off as the voltage across the b-e 
junctions drop below their minimum forward turn on voltage, the 
positive feedback drops below unity, the "SCR" resets and the cycle 
repeats.

13. FOr a trigger to occur, the average current from the Solar Cell 
must be greater than the average of the combined current pulses in 
the SE when the voltage on the storage cap at the SE trigger level.

14. For a reset to occur, the average current from the solar cell 
must be less than the average current through the SE and motor 
winding when the voltage on the storage cap is at at the SE reset 
voltage.   

And that is how a FLED SE works.