Originally Posted by Sheldon Brown's bicycle glossary

Donkey Back

A "donkey-back" is a setup for a multi-rider tandem, where the rearmost stoker's crankset is concentric with the rear hub. This stoker's saddle is mounted on an extension that resembles a pannier rack.
It requires a very special rear hub with an axle within an axle. Their is a synch chain running from the donkey back's cranks forward to the penultimate stoker's crankset.

Donkey backs were used primarily around the turn of the century as pacing vehicles (usually quads) for track races (a task later taken over by special motorcycles, such as the Derny). The donkey-back design placed the rear stoker back over the wheel, to give the racer on the solo the best possible draft.