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Transistor On: Current flows through the primary winding, creating a magnetic field.

Transistor Off: The magnetic field collapses, inducing a high voltage.

LED Glows: The induced high voltage causes the LED to light up.

Cycle Repeats: The process repeats rapidly.
In summary, the LED glows because of the high voltage pulse generated by the collapsing magnetic field in the primary coil of the Joule thief circuit.

Inductive Kickback: The process of the magnetic field collapsing and inducing a high voltage is often called inductive kickback. This phenomenon can generate a voltage much higher than the supply voltage.

Pulse Operation: The LED does not glow continuously; it receives high voltage pulses each time the transistor switches off and the magnetic field collapses. However, due to the rapid switching (hundreds or thousands of times per second), the LED appears to glow continuously to the human eye.

Energy Transfer: The energy stored in the magnetic field of the coil is transferred to the LED during the collapse of the magnetic field.