The back-transfer mechanism is used to transfer an intermediate result from the accumulator back to the rotor, for use in further calculations.

The mechanism uses a set of idler gears suspended from the intermediate star wheel shaft. The idlers are permanently meshed with the star wheels, but usually hang vertically at the rear of the carriage and take no part in normal operations.

Performing a back-transfer involves first clearing the rotor, then raising the idler gears to mesh with the outer gear teeth on the rotor setting rings. The setting rings are offset to the left of the star wheels, so the idler teeth are made wider to mesh with both. The idlers are raised by the lever at the right-hand end of the accumulator, and held in position by the small latching lever below.

The clearing handle is then turned, collecting the numeral wheels and driving them back to zero. The motion is transferred through the star wheels and the idler gears to advance the setting rings by the same number of positions. A cam on the accumulator shaft releases the latch at the end of the clearing cycle, dropping the idler gears back to their rest position.

This view is taken from the front of the machine, looking up under the rotor. The rotor is in its home position, with the setting rings at zero. The setting handles are visible at the top of the picture.

The wide idler gears on the carriage move upwards into the semi-circular cut-outs in the rotor discs. The gears are guided into engagement by a slightly elongated slot below the first tooth in the cut-out. The gears will not align with the cut-outs unless the rotor is in the home position, and the teeth will not mesh unless the setting rings are at zero.

As the accumulator numerals move back to zero, the idler gears pull the setting rings downwards by the same number of positions. The idler gears are released and drop away at the end of the clearing cycle.