"The most basic and important difference between internal and external martial arts is the method of generating power or "jing" (manifest energy). At the root fundamental level, the most important factor which qualifies an art as internal is the use of what the Chinese call "complete," "unified" or "whole body" power (jengjing). This means the entire body is used as a singular unit with the muscles of the body in proper tone according to their function (relaxed, meaning neither too tense nor too slack). Power is generated with the body as a singular unit, and the various types of energies (jing) used are all generated from this unified power source.
The external martial arts, although engaging the body as a whole in generating power sequentially, do not use the body in a complete unit as do the internal martial arts. The external styles primarily use "sectional power" (ju bu li), which is a primary reason they are classified apart from the internal arts. A variation of this sectional power in the external arts is the special development of one part of the body as a weapon (iron palm, iron broom, etc.). The internal tends to forego these methods in favor of even development of the whole body, which m turn is used as a coherent unit.
Xing Yi Quan, Tai Ji Quan and Ba Gua Zhang all have unified body motion as their root; hence, they are internal styles. However, since each of these styles emphasizes different expressions of this unified power, they are not the same style.
- Tim Cartmell, Internal vs. External Martial Arts