It is basically a multivibrator circuit. It rely on inherent imperfections on the circuit elements to aid its startup. This type of circuit will NOT stimulate properly in circuit simulators unless deliberate imbalance is introduced in at least one of the circuit elements.
What is meant by imbalance is the minor difference in one of the transistor gain, a couple of turns more in one of the coils of the center tapped winding or minor difference in resistance due to their tolerance.
Having said these, when the power is applied the collector current in top transistor row and bottom transistor row will race and one of these rows will dominate. During this time, the induced base current will be as such where it will increase this domination and cause the transistors in that row to go into saturation. This saturation cause the positive feedback base current to those transistors come to an end. At this point the other group of transistors will start to take over the domination. Now the magnetic field in the core switches direction, due to the way the connections are made. The growing magnetic field now induces base current to provide positive feedback to these transistors until the collector current saturates.
This ultimately makes this circuit self oscillate back and forth between saturation and cut-off of the two groups alternatively.
The speed of this oscillation, or the frequency of the oscillator is determined by the transformer impedances (RL value), the RC value at pin 5 of the base drive and also, a bit determined by the base resistance of the two groups of transistors.
This chopper circuit output voltage depends on the primary to secondary windings ratio. The control is open loop, thus voltage regulation is poor.