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The ancient texts and various writings, commentary and codifications about internal strength start out with the idea of Heaven as a source of power, the Earth as a source of power, and many having his own ability and "qi" with which to utilize and combine the qi of Heaven and the Qi of Earth. The "Superior Man" (as the classics say) will use the power of Earth and Heaven to provide as much of his needs and strength as possible. I.e, the "superior man" doesn't use brute strength, he uses qi in its varying format.
The Qi of Earth provides the support of the ground for us to use in place of strength and it provides the weight of an object (downweight) to do work that requires down forces. Said slighty differently, the earth provides forces upward from the surface and it provides forces downward due to weight. Both of these functions indicate that generally speaking, the Qi of Earth is mostly a description of Gravity. In a number of both Chinese and Japanese martial-arts styles, you'll see this reference to Gravity (Kuroda Tetsuzan hands out a flier to beginning students describing the importance of "gravity").
Up and Down, Open and Close, Yin-Yang, etc., are all dichotomies explaining the basic body's function in relation to Gravity. To "Open" is to expand and inhale, with the joints naturally winding openward... the strength of the solidity of the ground is expanded upward and outward; To "Close" is to exhale while the body sink and the joints wind inward... the weight of gravity is in assist. Moves of power that accompany this natural cycle of movement are augments because of the combination of forces.
There are generally 4 basic forces that a human can manage and manipulate using the solidity of the ground and the downweight:
Up, Down, Away from the Body, and Toward the Body. In Taijiquan, those four forces which utilize gravity and the solidity of the earth are called "Peng", "An", "Ji", and "Lu". Using the body to manipulate Up and Down powers, 4 powers can be arrived at and from those 4 directions of power, power can be made in any direction as the mind wills changes within the body to help aim the forces that come from ground-support and gravitic-weight (this is what "intent" means).
The supporting force from the ground is a straight-line force and so is the force from gravity. You can vector add these forces, but you can't curve them.
Movement in varying directions (lets' say moving one arm in a large circle) with the forces derived from groundsupport (let's call it the 'groundpath') and weight-force ('downweight') involves training the body and the mind to work together so that trhe clearest-possible forces of Up, Down, Away from the body and Toward the body are being conveyed to the hand/arm. This can be verified with feel. There is no true "winding" or "circular" until the body learns how to do this and does it with the aid of the dantien controlling the body.
2 cents. If this seems helpful to some people, I'll try to add onto as I can. Comments and critiques welcome *about the facts of the issue*, but if one of the usual suspects begins a personal attack, no more posts on the topic and other areas of internal strength.