Simply put, the Coordinated Effect of Three Body Parts (三合作用) means to combine the three parts of the human body.
All movements are achieved through 'arm (手), torso (身), leg (足)', where if these three areas were to be combined in a particular shape or form, it becomes possible to gain control over the conditions of weakness or strength in the internal body. The Daoyin movement forms were precisely designed to take advantage of this triad effect, where the very core of Daoyin can be said to be in the specific exploitation of this triad effect, and the hallmark of differentiation from any other forms of training.

By joining the root of the palm to the end of the foot, the kidney is able to breathe, for the Qi circulation effect to be stimulated.

The proper shape of the hand for each of the movements is an important point for achieving the Coordinated Effect of Three Body Parts.
In the Authentic Five Animals Qigong, the diverse hand shapes for each of the movements are clearly prescribed.

Qi operation stems from the Coordinated Effect of Three Body Parts, and the following explains this concept as an allegory. The precise moment when < light > & < an object > & a white < wall > are combined, a shadow appears automatically. Or, by the effect of the three becoming as one, a fourth emerged (shadow) by itself, where the movement of Qi is the same as this example. When the three parts of the human body are combined, from within the body occurs a Qi effect in conformance to the natural principle of Yin & Yang. Conversely, for any of the three elements of light, an object or the wall to disappear there would no longer the shadow itself to be, so in the same manner if in any of the movements arms, torso, or legs one element were to be left out, the flow of Qi would be disabled.

Examples from among the Authentic Five Animals Qigong training, of a movement which fails to achieve the Coordinated Effect of Three Body Parts.

In training for Daoyin, once the Coordinated Effect of Three Body Parts is understood in the stopped motion state, then the process of movement itself is emphasized. So in properly grasping the transformation of the body from one integral form to another integral form, the body comes to possess what is a sequential effect of the Qi.
The most important point in the Coordinated Effect of Three Body Parts is for each of the movements to be precisely mimicked with mastery over hand and foot, to benefit from the amplified effect of Daoyin. Once accuracy is achieved, it becomes important to maintain this suitable combined state even in the midst of the process of transformation from one move to yet another.

Movements which are the same on the outside can still tweak the Fullness & Emptiness (虛實) changes to the body from minute variations to influence the movement of Qi. For example, the problem of whether to place the weight on the toes or on the heels, or if to spread or close the fingers,these are issues which the instructors have to explain to the trainees. Since these questions over the Coordinated Effenct of Three Body Parts will not be answered from simply learning the external movement forms, they must be learned properly from teachers who have proper understanding of this triad effect and Qi operation. Only then will Daoyin be understood in its depth to include the Huatuo's Five Animal Qigong, to enjoy its benefits through practice, and to maximize its effect from training. Therefore, among the people who offer instruction in Daoyin, whether or not this Coordinated Effect of Three Body Parts has been mastered can be said to be a good litmus test for finding a good teacher.

Various movements of the Authentic Five Animals Qigong with clear examples of the Coordinated Effect of Three Body Parts.