I may be wrong, But i belive it refers to the trend of moving everything low and center, to balance the bike. the fuel tank motor, trans and most other major components are relocated to the center and bottom of the frame. Its similar to the character punching bags that you blow up for kids that have sand on the bottom. No matter how hard its hit it always comes back up
No "theory"...simple physics. Its a simple case of leverage and inertia: Mass located further from the center of gravity (balance point)requires greater energy (effort) to effect change (direction, deflection, acceleration, or braking) than mass located closer to to the CG. That's why racebikes get so tiny, relative to street bikes: faster turn in, more "road feel", quicker side to side transitions...there is a trade off, however. You lose stability. Negative feedback, weave and wobble, the tendency to wheelie, all become greater, due to the greater leverage of the feedback forces...that's why we end up with steering dampers, sprag clutches and electronic traction management systems on race bikes...they are "nervous" feeling, and any slight error will put you on the ground.
The art in sport streetbike motorcycle design is to make the bike as responsive as possible, without making it so unstable that you need an AMA expert license to take it to the local convienience store.