The concept of body steering relies on the rider's shifts in weight and pressure application on various parts of the motorcycle to turn it. Theoretically, one could turn a bike without ever touching the handlebars simply by shifting body weight. For example, to turn the bike to the left, the rider could simply stand up slightly and shift his/her weight somewhat to the left. Initially the motorcycle would compensate for the rider's motion by leaning and steering to the right, but would then countersteer into a left turn.
The relevant equation for this motion is
M=mvwhere M is momentum, m is the mass of the object, and v is its velocity. Once again, this equation is reliant on the conservation of momentum. (Sport Rider)
There are some racers who prefer one method of steering over another, but in my personal experience, everyone mixes the two. For professional racing, both deliberate countersteering and body steering are necessary to turn the bike in a racetrack scenario.