Originally Posted by Jonny B

There is (was) actually a device that replaces the original driver (the bit the cog fits onto and moves the clutch back and forth to drive or brake) with one with a freewheel thread, allowing you to fit a regular freewheel so you can back pedal. However, it's much easier to just dismantle the hub and take the brake shoes out. Just undo the drive side cone nuts, take the driver out (turn it clockwise as you pull to keep the clutch threaded on), then take out the brake shoes and slide the clutch and driver back in. Tighten the cones as normal (no play in the bearings but as loose as possible) and you effectively a freewheel hub you can roll back on without the cranks spinning.