Can of worms alert!!!
Crikey, I go and have a nice kip and all hell breaks loose! Never mind, the body position/steering question is worthy of study although the subject is vast in scope.
First things first, let's take a look at assisting the steering of the bike by 'weighting' one or other of the footrests.
For all intents and purposes a bike and rider forms a 'closed' weight system and apart from weight loss due to fuel burn, sweating and tyre and brake pad wear, what weight you start out riding is all you have got to play with. (the picky amongst you will notice that I haven't factored in any increase in weight due to G forces, but as G increases/decreases effects all the loaded weight equally it can be ignored for now)
You cannot ADD weight to such a system so to make any change in the balance (CCofG) of the bike any loaded weight has to be moved.
You can put as much 'weight' (pressure) on a footrest as you like, but unless the rest of your body moves, it will have absolutely no effect.
There is a simple experiment you can do to prove this the next time you are out on your bike. Simply ride in a straight line (the bike isn't a sentient being, so it doesn't know whether it is upright or banked over) and keeping the rest of your body still, press as hard as you like on one or other of the footrests and apart from a tendency to lift your butt off the seat, absolutely nothing happens. Repeat the experiment only this time take your feet off the footrests and move your head laterally to one side or the other and hey-presto, the bike starts to turn (albeit slowly) in the same direction.
Making a bike steer requires that the rider displaces the CCofG to one side or the other of a line connecting the two tyre contact patches thus forming an out of balance couple. This can be done in just a few ways which are turning the bars, laterally shifting the CCofG (or having it shifted for you by an external force such as an uneven road or gust of wind) or changing any centripetal forces acting on the bike during a turn by varying the throttle or the brake. Turning the bars has by far the biggest effect on the relative position of the CCofG, but in Moto Gymkhana the other two methods play a crucial role especially at slower speeds.
The steer with one hand method as detailed by Lee Parks certainly has a beneficial effect on our riding because our bikes have a dirty great hinge in the middle which must be allowed to move freely during the steering process. A little acronym that you should remember is FAST or Fear+Anxiety+Stress=Tension. Any tension in our arms and upper body reduces the ability of the bars to move freely and instead acts like a form of crude steering damper which slows the steering rate down. By reducing the fear, anxiety and stress we feel we automatically reduce the tension which improves the steering which helps reduce the fear and so on in a virtuous circle. Steering with one hand goes a long way towards this pleasent state of affairs as it reduces any undesireable forces on the steering by 50%!
To further reduce the undesireable forces acting on the bars, our Japanese friends promote a basic body position of Butt Back, Knees In, Elbows Out and Head Up. With the elbows out there is much less likeleyhood of one or other of your elbows crashing into your upper body as the bars turn. To see what they mean try this experiment whilst sat at your computer. Hold both hands out in front of you as you would if holding the bars and point your elbows down. Now move one of your hands back towards you until your elbow hits your upper body and note the position of your hand. Repeat the experiment only this time stick your elbows up and out and this time you will see that your hand comes much further back towards you than was previously the case. Notice also that with the elbows out the wrists become bent somewhat awkwardly on the grips, so you will need to change the angle of your grip from 90 degrees to about 120 degrees. At this angle the grip no longer lies along the line of your knuckles but follows the creases in your palm instead.
Vulfy wants a discussion on EVERYTHING which is quite a big ask, but it's certainly worthwhile having the discussion as there is a great deal to learn.
Dedicated to the wonderful sport of Moto Gymkhana