Every motorcycle rider knows that low-speed maneuvers are the most difficult ones. That's one reason why the Motorcycle Safety Foundation's Basic RiderCourse focuses so much on them. The gyroscopic effect of the wheels stabilize the bike at higher speeds, but without those forces, anything on two wheels is quite likely to tip over. That's why motorcycle trials are an even more difficult test of rider skill than any MotoGP or Supercross race.
Precision, rather than outright speed, is the goal of a trials rider. It takes a great deal of control to balance the bike while stopped, and an insane amount of control to ride over obstacles that would be difficult to climb even on foot. But that's exactly what these riders do, seeming to defy gravity and all laws of physics in the process.
Neither boulders, nor boxes, nor dumpsters stay these trials riders from the swift completion of their appointed rounds. At times spotters climb the obstacles to help the riders out. They hold their hands out to provide a visual reference of where the bike needs to end up, which the riders sometimes can't see from their view on the ground. It takes a literal leap of faith to accelerate and jump up an object without being able to see where you're going to land until it's too late to alter course in mid-air.
These riders pull off some other seemingly impossible moves, such as backing up or turning around in a space barely bigger than the bike. Trials bikes are made specifically for this sort of insanity. I'm guessing that author Douglas Adams' Infinite Improbability Drive is standard equipment, which would explain how these impossible maneuvers become possible.