No center of mass moved forward is less then center of mass moved up. Your putting the majority of your body weight above the tank, you raise the center of the bike up. Its the same thing as lowering the bike vs raising the bike. If you lower a bike 3 inches, you lower the center of gravity 3 inches, you put it more in line with the axles. SO when you put your weight on the bike you are putting the center of gravity even further above the axles. The higher the center of gravity above the axles the less mechanical advantage there is. Much like picking up a piece of ply wood, if you pick it up flat in the 4 foot direction it is hard on the bottom and gets easier the higher it gets. If you pick it up the 8 foot way it is much heavier on the bottom and at the top its just as easy at the 4 foot way.
Imagine a line from the back axle to the front of a bike. Now you take the center of gravity and draw a horizontal line through it from the front of the bike to the rear of the bike. Now draw a vertical line through the balance point of the bike front to rear. Connect the 3 lines and you will have a angle off the rear wheel going upwards to the front. With no weight on the bike that angle would be fairly shallow. Now add your weight to the bike and you move the center of gravity up, and the center of mass forward or back depending on many factors. Lets say you can lean forward enough that you do move the balance point forward (I doubt many can) Now draw a new line off the rear axle to the new higher horizontal line and the new vertical line and you will see the angle of that line is much steeper. That line is the same as lifiting a peice of wood, the steeper the line gets the easier it is to pivot on the rear axle. Stretching a bike will cause your angled line to be shallower by moving the balance point between the axles forward. Lowering a bike cause's the horizontal line to be lower and shallower. When you lay down on the tank, most think you are loading the front tire (in some cases like really tall thin ppl it might be true), the bigger truth is you are lowering your center of gravity.
I suck at explaining crap, and its been years since I did physics but thats kinda the basics of it.
Moving weight forward does help, that's why longer swing arms work. Weight is further ahead of the rear axle, the Brock's brick helps so much cause weight is further ahead of the rear axle and on the same plane.
Its all that silly physics crap, I didn't invent it or i would have done it a little different
