Assume that when the DC is applied to the given circuit, the T1 transistor is conducting with the current passing through the N2 and R1 resistors at the first moment. As the current from R1 flows from N1, it can not reach its maximum value immediately. (The maximum value is only after 5 t.)
As the current passing through N2 rises to the maximum value, a high-value current starts to flow from coil N1 because the T1 conduction passes. The variable magnetic field generated by the current flowing through N1 induces a voltage in coil N3. In addition, it also weakens the magnetic field formed in coil N2, bringing the current through the N2 winding to a higher level. When the currents passing through the windings N1 and N2 reaches the saturation (maximum) point, the magnetic field around N1 stagnates.
The stagnation of the field of N1 nulls the tension in the second. In addition, the pressure generated by the N1 coil on coil N2 is removed and the current of the N2 begins to decrease. When the current of N2 decreases, this time a magnetic field force is generated on N1 opposite to the previous one. The inverse magnetic force generated at N1 again
acts on N2 again to suppress the current flowing through N2 to zero.
The zero crossing of the current through N2 also makes the current through N1 zero. In this way, the circuit is back to normal. Then the small valued current passing through N1 drives the T2 transistor. The circuit continues to operate as described just a moment ago.