Anyone who's taken a spill from monkey bars and wound up with a nasty bruise or a broken bone knows that the impact of a soft body on a hard surface is no picnic. There are lots of factors at play when a body meets pavement, and a very important one is energy.
Take a daredevil making a leap from a rail on a building. The height of the rail is the basis for his potential energy -- he's resting, but because of gravity, his distance from the pavement gives him a stored energy source. When he makes the jump, that potential energy becomes kinetic energy, the energy of motion. But the energy sources don't stop there: When he pushes off the rail with his feet, he increases his speed, adding to his kinetic energy.
Landing the wrong way from such a stunt could be disastrous, in large part because all this energy still exists when the jumper gets to the ground. If he does nothing to get rid of it -- say, if he lands flat on his feet with his knees straight and his muscles tensed -- that energy will expend itself however it can. Most likely, this will involve some broken bones.
Fortunately, a free runner has lots of ways to help his body follow the law of conservation of energy, a basic law of physics explaining that energy can't be created or destroyed. If he bends his knees and rolls into a somersault when he lands, he's allowing more of his body to absorb the energy from the landing. And, he's expending more energy as he moves, cutting down on the odds of a sudden stop and a broken bone.
