This extract is from an article about the relationship between tanren and other arts (Battodo is discussed here and the practice is relevent to any art) by John Evans in SMAA Journal Vol. 14, Issue 3.

Any arduous progressive training may be termed tanren, but tan and ren, the Chinese characters that make up this word, refer originally to the processes involved in forging metal. In the context of learning a physical art, tanren refers not so much to the acquisition of skills as the transformation of structure. Both kanji refer to forging, refining, or tempering of metal thus encompassing every phase of sword making. Despite the obvious differences between metal and human tissue, there are many parallels between the phases of sword making and the stages in the making of a swordsman.

Three Goals of Forging

Purification, homogenization, and integration of soft and hard qualities are also the goals of the forging of the budoka. In the Chinese martial systems this process is called nei gong—inner work or chi gong—energy work (kikou in Japanese). These terms are used to differentiate “internal” training, which focuses on internal energy from “external” practice that focuses primarily on muscular development. In practice the distinction between internal and external is not clear-cut, since all internal transformations are expressed through the physical body.

Inner work usually begins by slowing down and in other ways “loading” the movements of one’s art whilst coordinating them with slow breathing. If the mind is properly attuned and can pick up the appropriate cues from inside, this leads to a conscious engaging of tissues, organs, and systems normally considered outside the control of the mind. This attuning of the mind is the crucial difference between the forging of metal and the forging of the budoka. Even with a sound system and a skilful teacher, everything depends upon the mind of the student. At each stage the mind must participate appropriately for the steps of transformation to be successful. There are no manuals for this process since the internal cues and the sequence of steps will differ according to the unique constitution, physique, and mindset of each student.