There is a slot in the backplate that the pin catches, this is what turns the crank shaft when using the pull start. Put the pin in gently and then try to line up the backplate. Do not use to much force when putting the pin in or it will fly out into the unknown. When you put the backplate back on ,pull on the pull start and see if the crank moves. If the pull start spins and the engine doesn't the then pin came loose. Kind of feels like the one way is slipping!