Tedkx, has it right, the main reason the landing is so much smoother when
the landing is angled rather than flat, is Gravity. This mysterious force is
practically useless when the bike lands nose down and matches the
landing face.
One thing to keep in mind, it is most important that your bike match the
angle that you are landing on, you can still land on an angled ramp, and have
a very hard landing if you land on the rear wheel first, let off the gas and
let the front wheel slam down. OUCH!
When your up in the air 10 to 15 feet, and you land on flat ground, you have
gravity pulling straight down, all 400 LBs. But, when you land on an angled
surface correctly, the downward pull of gravity is almost eliminated.
Note: This mysterious force does not seem to work on "Bubba" Stewart,
I have seen him jump a 65' foot triple, completely overshoot the landing,
land completely flat, and just keep rolling, absolutely amazing!
I have seen other people (Mortals) do the same thing, and them and their
bike bounce completely off the track!