When DC is applied to the given circuit, suppose that the T1 transistor is transmitted with current passing over N2 and R1 resistance. The current from R1 cannot reach the maximum value immediately when passing through N1. (Maximum value is only after 5 t.)
As the current flowing through N2 increases to the maximum value, a high current flows through the N1 coil as the T1 is transmitted. The variable magnetic field generated by the current passing through N1 induces a voltage in the N3 coil. In addition, it weakens the magnetic field in the N2 coil and increases the current passing through the N2 winding to a higher level. When the currents passing through the windings N1 and N2 reach the saturation (maximum) point, the magnetic field formed around N1 becomes stagnant.
The stagnation of the area of N1 reduces the voltage in the secondary to zero. In addition, the pressure created by N1 in the N2 coil disappears and the current of N2 begins to decrease. While the current of N2 is decreasing, a magnetic field force is created on the N1 in the opposite direction of the previous one. The inverse magnetic force at N1, which again affects N2, pushes the current through N2 to zero.
The current flowing through N2 to zero also makes the current through N1 zero. This way the circuit is back to the top. The small value current passing over N1 then drives the T2 transistor. The circuit continues to operate as described previously.