best guess is they just reverse the firing and custom map the ecu. I'm trying to figure how well the water pump and oil pump work in reverse though. I guess you can run those externally to the motor by an oil sump and an electric water pump. Then you have the tensioner that may cause some issues going backwards.
However I would not try this with a used engine, i would do it on a fresh build, there's the potential issue with wearing on the bearings because of the load being reversed, then again i cold be completely wrong.