well, as we saw back in the day with Evil Kenevil, it's usually the landing where it all goes wrong...
physics-wise, Evil used to land on some very flat ramps, and would get kicked off a lot before the energy was dissipated, as he didn't really have a way to absorb the energy...
he was dependent on coming in at a smooth angle (with very little margin of error), and carrying on down the ramp...
any bump and he was done...
here the curve of the ramp acts to absorb the energy...
the angle of entry still has to be right...
too much forward, or backward, lean would spell disaster when you hit the transition, but the margin off error is better than it was for Evil...
he gives it enough gas to come of the roof with control, leans a bit forward, and lands near vertical on the ramp...
rolling through the transition (curve) on the ramp distributes the energy smoothly, so you don't get bucked off...
the shocks of a dirtbike help a lot compared to the road bikes Evil was riding 40 years ago...
it would be an interesting curve when seen from the side...
i bet that the force of gravity would be the biggest player in any equation, compared to the raw power of the bike as in a normal freestyle moto stunt...
normally the guys have plenty of time to gun the engine, or brake, while in the air to correct the bike's flight...
this one is more about building the correct size ramp, with the right amount of transition, rolling off the roof with just enough speed to get off the roof clean and line up with the ramp, and let gravity do the rest...
fairly simple from a physics point of view...
giant, hairy, balls of steel from the human perspective

...
cheers
jah