The principle is rather complicated, so I'll leave it at that.

This circuit consists of an oscillation circuit and a switching circuit.

The oscillation circuit consists of transistors V1 and V2, resistors R1-R4 and capacitors C1 and C2.

The switching circuit consists of diode VD, resistors R5, R6 and transistor V3.

The ultra-low-frequency signal generated by the oscillation circuit is added to the base of V3 via diode VD and R5. When the steering switch S is placed at R or L, V3 conducts intermittently under the action of the ultra-low-frequency signal to make the turn signal HL1 or HL2 flashing light.