"Every action has an equal and opposite reaction ." Sir Isaac Newton 1687.

Preloading is simply the act of crouching before a leap , and is relied upon heavily in trials riding to build/accelerate momentum. By lowering the body and compressing the legs, a rider can explode upwards/forwards more explosively .

Interestingly, to some extent riders can transfer this effect to their bikes by compressing (thus loading with kinetic energy) the rear tyre and in a small way using the flex inherent in a frame.