Have a look at the transmission on this old bike:
See how it works? Might be clearer on this diagram:
You pedal forwards normally to use the high gear, and if needed pedalling backwards uses a much lower gear.
Quirky and simple French engineering.
A loose translation of the ad below is - "Different ways to get up hills: smiling, with retro-directe; sweating blood and tears, with a single speed; or on foot, pushing your bike!"