Hello Kelvin, very good question.
Drives that have this capability in their firmware take advantage of the inertia in the load the motor is coupled to. When power loss ride through is enabled it simply decelerates the drive enough to keep the bus voltage charged above the shut off point. All AC drives will pump energy back into the DC BUS if decelerated faster then the load will coast down by itself. So with Power Loss Ride-through enabled, the drive does not fault as soon as the input power shuts off during run mode. It just decels it fast enough to keep the BUS at a level to keep the drive alive.