In theory, it's possible that the prop could actually spin in reverse direction. If that were to occur, the engine would actually produce gas. On a long tow (in excess of 60 miles) the gasoline produced would be considered to be untaxed and in excess of what is legally allowed for self consumption. I'm sure others will be along who can speak more intelligently on the issue than I can, but I think both Federal and state fuel tax laws are in play.