Some people on LPG have to run it backwards if they haven't installed a balancing pipe - essentially, if they are driving along and (let's say) take the foot off the gas as they are going down hill, air is still being rammed into the engine at a higher rate with reduced LPG due to no throttle - basically causes the engine to snuff it .. no fun at 80 or 100 k's per hour!! That's only one reason, I'm sure a lot of people do it thinking it looks cool (as though they go through water that deep that the snorkel has to be backwards)!! lol - probably a few other reasons also ...