Originally Posted by
R_Garrelts
Lifting the legs only seems more impressive if you haven't thought about the physics involved. In actuality, in the demo you are describing, any downward force applied to the ground by the legs serves only to create a torque in the same direction as the torque resulting from the pusher's force. For Ueshiba to remain (rougly) fixed in place, the sum of the external torques must have been (roughly) zero (though we can, of course, quibble about the exact direction and magnitude of the pusher's force). Therefore, it would have taken less force to topple him if he didn't lift his legs. Extending the legs forward also moves one's center of mass forward and, therefore, serves to increase the torque due to gravity (the torque that directly opposes that of the pusher).
If you think about it, it's rather simple: when you are sitting on the ground with legs outstretched in front of you, the parts of your legs which are in contact with the ground are applying a torque to keep you in place under the influence of gravity. When someone starts to push on you, though, they are now applying that torque and less force is required of your legs to maintain equilibrium.
I suspect this is at least part of the reason why so many people believe the pusher's force to be "channeled" somehow into the ground (as in the often used analogy of an electric current): If the person being pushed wishes to remain in place, both the net external forces and torques must be roughly zero (otherwise, there would be appreciable translation or rotation, respectively, of the person's body). But this means that, if one is initially in a forward stance, for example, the forward foot must become "unweighted" in response to the pusher's force, and, therefore, an increased vertical force must be applied by the rear leg (to prevent the body from translating vertically). The increased weight on the rear leg is then responsible for the sensation that the pusher's force has been routed "into the ground."
Another trick you can add is lifting under the arms of the pusher (per some of Gozo Shioda's demos). The effect is, again, to increase the torque in opposition to that of the pusher, since lifting up on the arms tends to make one's body rotate forward.