It is based on a pulse trapped in a laser resonator, which stays in there until it extracts all of the energy stored in the amplification medium.
Pulse trapping and dumping is done using a polarizer and a Pockels cell, which acts like a quarter wave-plate.
If no voltage is applied, then a double pass through the Pockels cell will not change the polarization and the pulse will get trapped inside the cavity of the resonator.
The pulse can stay in the cavity until it reaches saturation or until it extracts most of the energy stored in the gain medium.
Regenerative amplifier can also operate at Radio Frequency,[1] using the feedback between the transistor's source and gate to transform a capacitive impedance on the transistor's source to a negative resistance on its gate.