Retro-Direct is a gearing mechanism that was included on some bicycles built in the early 20th century. Retro-Direct allows a different gearing when the cyclist pedals backwards. Some bicycle hobbyists have built Retro-Direct onto modern bicycles.
The first version of Retro-Direct used two separate chains, however a single chain version soon followed. The single chain system is actually quite simple: The chain goes from the top of the pedal sprocket to the top of the rear wheel sprocket as usual. However, the chain then wraps around the wheel sprocket to an idler sprocket located between the wheel and the pedal, and then back to a secondary sprocket on the rear wheel, and finally back to the bottom of the pedal sprocket. Since the chain wraps around the secondary wheel sprocket in the opposite direction as the primary wheel sprocket, the cyclist only needs to pedal backwards to engage it. Usually the secondary sprocket is larger, which provides the cyclist a lower gear for climbing steep inclines simply by pedaling backwards. The two rear-wheel sprockets have separate freewheels, which the chain turns in opposite directions. Whichever is turned forward is engaged, while the other spins freely backward.
For clarity the drawing shows a system where the primary wheel sprocket is larger and gives a lower gear than the secondary wheel sprocket; pedalling forwards uses the lower gear ratio.
For the more common system where pedalling backwards uses the lower gear ratio, the drawing should show the chain from the small wheel sprocket going to the top of the chainring (pedal sprocket), continuing around the chainring, and ending up on the top of the large wheel sprocket. So to climb a hill, you pedal backward, and to go fast you pedal forward. While most historical examples of retro-direct bicycles used the reverse gear for climbing, several modern retro-direct riders prefer climbing while forward pedaling and having the higher, cruising gear being the one that is pedaled backwards.