Been, done, T-shirt.
With one leg (and gord only knows how the poor sods with NO legs manage), anyway, with one leg you have to put your GOOD foot on the concrete, and then push the pedal down with your plastic foot. Which works, a lot of the time, but then you have to have got enough momentum to get your good foot on its pedal, too, and then pedal away.
This has drawbacks. Overweight rider, or a slightly UPHILL start, and you are cattled. Just cannot get enough push with the plastic one to get to that 2mph you need to maintain level flight. I used to push my bike round corners to find a bit of flat road, then start off and hoped to get up enough steam to go round the corner onto a hilly bit. Usually worked.
Anyway, Kalkhoffs were fine and powerful, and very nearly addressed this problem. But not quite.
(Why start with plastic foot on pedal? Because without feeling, it's almost impossible to position your plastic one on a MOVING pedal!)
Allen.