Nelson White's U.S. patent "Zero-setting mechanism", filed in 1920, describes how this amazing thing works.

The clutch rotates one gear forwards when the crank is turned forwards, and it rotates another gear backwards when the crank is turned backwards.

One gear turns the upper shaft, the other gear turns the lower shaft. Little arms on the clutch lock the shafts into place when they are not turning.

On the left side of the carriage, cams on the upper and lower dials move a lever on the central zeroing shaft. Fingers on the zeroing shaft as a result move into place so that when the upper or lower shaft is rotated, the wheels stop when they display zero.

The zeroing mechanism rarely completely fails. It is useful to rotate the crank while observing all the different parts to gain an understanding of each part's function.