QUOTE(Mike Sigman @ Aug 11 2008, 12:49 AM)

But how does the hara get it's power? From the stability of the ground through the legs, mainly (I could get more complicated and bring in the weight and a few other things, but I'm trying to keep this simple). In other words an articulate and very powerful middle is needed if you truly want to move from the hara.... and the legs must provide the strength and help with the articulation. Now, I'm aware that someone who doesn't understand this topic is going to miss or not understand the implications about "jin" forces (perhaps cobanhutton can give his impressions here), so I'm not going to go into that aspect since the general reasoning for "leg strength", with or without jin power, has been laid out.

Thank you for your confidence that I understand this well enough to offer useful impressions. :)
My own experience of "leg strength" and "using the hara" is that when one is standing with internal strength, the legs feel relaxed but sturdy, like mobile pillars upon which the hara rests.
The next step, in my experience, is that it is impossible to actually
move while maintaining an internally-strengthened stance without the hara functioning as the driver of the movement.
As a very simple example, consider the problem of resisting a push while standing in an internal way.
If I stand in a natural posture with internal strength "engaged" (for lack of a better word), my abdomen naturally sinks and kind of tenses in a relaxed way (if this makes sense). If someone pushes me, and I try to maintain my stance while maintaining this "engaged" feeling, my hara pushes back against the incoming force.
In actuality, the legs and back are doing the pushing -- but it doesn't feel like it. If I want to push harder against the incoming force, I have to push with the hara, or I lose the feeling of "engagement" and end up relying on external power.
From a biomechanical point of view, I suppose that what's happening is that the lower abdominal muscles are controlling the alignment of one's spine and hip joints in such a way that the large muscles of the back and legs are absorbing the force of the incoming push.
These muscles are so powerful in relation to most pushes that they hardly even feel the push -- the incoming force is felt primarily in the sole of the opposite foot, as if the force was being channelled directly into the ground. It feels as if the ground itself is supporting the force of the push. I think this is what the Taiji people mean when they say "receive the strength of the Earth," etc.
So, personally, I have learned whatever I know about the role of the hara indirectly -- by trying to push, move, etc. while maintaining the feeling of internal "engagement" or connection, and finding that it feels as if the hara has to be the origin and controller of the movement.
However, I must confess that I can't do anything but very simple things with internal strength (standing, resisting a push, simple pushing or, less effectively, pulling). More complicated movements require the kind of training one sees in Taiji, Bagua, etc. I imagine it takes a lot of abdominal strength and a rather highly developed sense of motor control in this area to really use an internal martial art effectively.