The LHS 2k resistor start the turn on the LHS PN2222, the 10k res gives turn on hysteresis, the.
The LHS PN2222 is on, this turns on the main xtors, the.
The output rises until the RHS PN2222 turns on which lifts the emitter R, turning off the LHS PN2222 as its base is fixed to 2v5, sometime.
Sometime later current flowing L to R thru the 39k and 1uF cap turns off the RHS PN2222 - as well as the o/p volts falling due to load, and the process starts again. If
If the load is removed suddenly the Vout will go up a little and then the RHS PN2222 will keep the converter off until Vout falls below design - set by the res divider on the RHS PN2222 - to give 2v5 on the base = 5V out here.