Regulating the breathing means to regulate the breathing until it is smooth, abundant, uniform, and fluid. To regulate it, the Yì is the most important. When the Yì is strong, (the circulation of) the Qì is strong. When the Yì is peaceful, the Qì can be led smoothly. When the Yì is calm, the Qì can be uniform, smooth, and fluid. Qì is bioelectricity. Yì is the electromotive force (EMF) (i.e., electric potential difference). From Ohm's law, we know that:
V= I x R
V=Electric Potential Difference I=Current
From this, we can see that when the electromotive force is strong, the current will be abundant and when the electromotive force is weak, the current will also be feeble. In addition, when the resistance is high, the current will be low, and when the resistance is low, the current will be high. Consequently, when the Yì is strong, the Qì is strong, and when the Yì is weak, the Qì is weak. When the body is relaxed, the resistance is low, and the current is high, and when the body is tense and stiff the resistance is high, and the current is low.
調氣者，調氣之行順、沛、均、與勻。其調之，以意為先。意 強，氣沛。意寧，氣順。意平，氣均勻也。由今之科學上之瞭 解知，氣者，生化電也。意者，生化電流之電動勢(電位差) 也。由歐姆定律知:..
由是可知，電動勢強，電量則沛;電動勢弱，電量則竭。不但 如此，電阻高，電量小;電阻低，電量大。因之，意強，氣 強;意弱，氣弱。身體鬆軟，電阻低，電流高。身體繃硬，電 阻高，電流低。..
The purposes of Qìgōng training are to increase the flow rate of the Qì's circulation (i.e., abundance of I) and also to improve the quality (i.e., efficiency) of the Qì's manifestation. From the formula, you can see that in order to increase I, you must increase V and also decrease R. In order to increase V, the Yì must be more concentrated. In addition, in order to decrease R, the body must be relaxed and soft. This is the theory of Tàijíquán: "relaxed physical movements with a concentrated mind." This is a way of "moving meditation."
The way of strengthening the Yì is through meditation training. The body relaxes through training the postures. Wǔ, Yǔ-Xiāng said: "Transport Qì as though through a pearl with a 'nine-curved hole,' not even the tiniest place won't be reached." To reach the level of transporting Qì to everywhere as wished, the Gōngfū of Yì's internal vision must be high and deep. Only if the Yì can reach everywhere (in the body), can the Qì be led everywhere. This Gōngfū must also be gained from training Qì in still meditation. Furthermore, in order to gain the uniformity and fluidity in Qì circulation, the transportation of Qì must be soft and smooth. The key is in the uniformity and fluidity of the breathing, the body's relaxation and softness, the Yì's peace and calmness, and the spirit's condensation and upraising. The Song of the Spiritual Origin of the Great Dào sings: "Concentrate (your) attention to reach the Qì's softness, the spirit will stay for a long time. To and fro of the real breathing will also be natural." This is the Dào of the unification of the spirit (i.e., Shén) and the Qì.
意之強，由靜坐練之。身之鬆軟，由練架得之。武禹襄云:〝 行氣如九曲珠，無微不到。〞為求氣之無所不到，意之內視功 夫必須高深。意之無所不到，才能導致氣之無所不到。此功夫 亦由靜坐煉氣中得之。再者，為求氣之均勻，氣之行必須柔 順。其要在於呼吸之均勻，在身體之鬆柔，在意之寧靜，在神 之內聚與上提。《大道靈源歌》曰:〝專氣致柔神久留，往來 真息自悠悠。〞此是神氣相合之道也。..
In order to increase the electric potential difference, you must first know how to increase the concentration of your Yì. The best way to increase concentration is through still meditation training. In addition, in order to reduce the resistance of the body, you must know how to relax and be soft in your movements. When you are soft and relaxed, the Qì can flow smoothly without stagnation. In order to increase the efficiency of leading the Qì with the Yì, you must have a very sensitive inner feeling. The deeper the feeling, the more powerfully your mind can lead the Qì.
In addition, when Qì is flowing in your body, it should flow smoothly and feel natural, which allows you to relax continuously, which in turn makes the body more transparent to the Qì flow. It is known that when your breathing is smooth and correct, the Qì's circulation can be smooth and natural. When you can regulate your mind and Qì until they are harmoniously coordinated with each other, and are coordinated with correct breathing, the Qì can be led effectively and efficiently. This is one of the main goals in Tàijíquán training.
The above is an excerpt from Taijiquan Theory of Dr. Yang, Jwing-Ming, The Root of Taijiquan, Second Edition, Publication Date May 6, 2023, YMAA Publication Center, 9781594399022.