As a former BMXer, I can only credit their skills and vestigial commonsense. In one of our less lucid BMX moments we got a big ramp and pointed it at a bigger vertical wall. The aim wasn't the jump the wall, oh no, that would have been too easy, the aim was jump at the wall and hit it at such an angle and speed that we could continue to pedal up the wall, sticking up two youthful fingers at the miserable clutches of gravity. That worked about as well as expected. Well, had we expected one of two outcomes (neither of which involved any kind of victory over gravity) – either bouncing off the wall if the tyres hit first or splatting against the wall otherwise. We spent an entire afternoon attempting to defy gravity. I think we only stopped through the effects of cumulative blood loss.
I snapped that Mongoose's frame riding off the garage roof.