Internal/External - Hard/Soft.

When I began Karate training I was uncertain of things, but I was also a child, yet when I took up Zenkutsu Dachi to hold the punching bag for my Dojo mates I was known to have the most solid stance among the juniors of that class.
Funny thing about it was I just kindof let it settle without even thinking about it and the force went through me into the floor... I was doing then, without thinking, what Wilder Sensei encourages Karateka to do now through Sanchin Principles outlined in his book 'The Way of Sanchin Kata.'

As a Karateka moves up through the ranks we tend to assimilate to those around us, to our Sensei and our Sempai, and things become less natural, more forced until one day the pendulum starts to swing the other way, if it is allowed to do so.
There is a reason for the way things are done in a Dojo, most times it is easier to teach everyone in a uniform manner, and this is often the best way to approach basics, if one knows what they are doing, but often no one ever moves beyond this point.

Going through some Kihon tonight, after going through some Kata, I was looking deeply at the mechanics behind what it was I was doing and was reminded of those days back in the junior children's class at the Olympic Martial Arts Center when I would just settle into the ground.
Instead of 'step then punch' I began to move in a way where the body moved just a couple seconds before the arms and the lead foot and the punches landed at the same time, sinking into the ground before drawing up from the ground again to move.

Keeping Weight Under Side involves more than tension, in fact, tension itself is counter productive to the principle and tends to take away power from technique as the mechanics become jumbled.
These also relate back to allowing things to flow through and sink - with tension things tend to become stuck, the body becomes broken, the focus, incomplete.
All is Sanchin, as the old saying goes, but more to the point, all is in timing and direction of intent, for lack of a better way of putting it.

The broken way of moving is an instructional tool to lead the way and must be left behind once a student starts to get the hang of things within the first few months.
Just as the Kata point beyond themselves to something more unified, universal, and personal, so, too, the Kihon point beyond themselves to the same thing, it all comes to a head when one point is achieved... Expressing back through the avenue of Kata makes the experience entirely different from this end of the spectrum.