I'm downloading the scene right now but i believe that the thruster is there to counteract the force the engine makes. the ball wants to stay in place but it is forced to go downwards, hence the thrust generated upwards.
P.S I started typing this when i pressed the download button and it didn't even start downloading. not even now. nor now. still not. Damn
It wasn't designed as you see it here. It originally had a fork like normal bikes but the body covered most of it up. I just added this weird forkless design because I tried a fork and it gave too much trouble
And yeas, I've ridden a few motorbikes and I know this would be hellishly twitchy and unstable at high speed