from of the chapter "CONTROL AND POWER: Kyujutsu":

In this traditional specialization of bujutsu, the following qualities were considered of fundamental importance by the teachers of the art: independence of vision, embracing as vast a field as possible; a keen perception of significant detail, without dispersion of attention due to fear or confusion; and power – power to draw back the huge bow, to control the release of the arrow and its trajectory toward the target. Regarding the first two requisites, kyujutsu doctrine made a clear distinction between the idea of aiming at a target (monomi) and that of concentrating and stabilizing the sight (mikomi); in the lexicon of this art, concentrating and stabilizing were general and diffused, while aiming was specific. The archer had to be able to enlarge or to narrow his field of vision and attention at will, in order to be aware of and able to control his environment as a whole. At the same time, he had to be capable of perceiving a particular shadow lurking in his vicinity, or even a single small chink in the armor of a galloping foe charging toward his lines on the battlefield. In this context, the art of archery availed itself of doctrines concerning mental control which were already ancient when they reached Japan from India together with the first manuals on Buddhism. The Japanese knew by heart the story of Arjuna the archer, for example. Invited to a tournament in which skill in archery was tested by having the archers aim at the eye of a painted wooden fish set high on a pole, many contestants were asked by a teacher, before they released their arrows, what it was they saw. With one exception, they all answered, "a fish." Arjuna replied that he saw only the "eye" of the fish, and, as might be expected, he alone hit the target. In order to develop this capacity to see clearly the whole and all its parts, kyujutsu made abundant use of haragei, "this art of the belly that runs through all the arts of Japan and whose mastery is a sine qua non in every one of them" (William R.B.Acker "Japanese Archery". Tokyo: Charles E.Tuttle Co., 1965).

Exercises of meditation and abdominal concentration, often performed by archers in the same monastic halls where spiritual leaders employed those same exercises for other purposes, such as mystical enlightenment (satori), were customary in feudal Japan. Such training was intended to produce a warrior who could calmly and inexorably release all his arrows against selected foes, even in the midst and clamor of battle, or when facing a horde of enemy cavalrymen rapidly converging upon him.

In relation to the power used by the experts of kyujutsu in handling their powerful bows, even the most modern manuals concerned with this art cannot avoid referring to the concept of ki (often using the more archaic and authoritative Chinese denomination, ch'i). Acker, who wrote a brief introduction to Japanese archery, refers to this "nervous" or "plastic" energy which "runs along our nerves from one part of the body to another like electricity along a wire" (Acker). This energy, for which it is difficult to find an exact definition in the English language, could presumably be developed by appropriate exercises for effective use in kyujutsu. "Systematic breathing," centered upon the hara, was naturally considered "the most powerful" exercise of all (Acker) because the relationship between the energy of ki and air or breath, that "lord of strength" (Robert W.Smith "Secrets of Shaolin Temple Boxing". Tokyo: Charles E.Tuttle Co., 1964), was considered to be one of substantial identity. Even today, practitioners of the modern derivation of kyujutsu known as the "way of the bow and arrow" (kyudo) are taught the primary importance of abdominal breathing in the development of power. This exercise, Murakami Hisahi tells us, is called ikiai, adding that "every action [in kyudo] should be synchronized with the rhythm of breathing" (Black Belt, monthly magazine, April, 1967). All kyudo teachers stress this idea of abdominal centralization as the prerequisite for extending the body in full coordination of intention with action, of will with respiration, and of both with every movement in the practice of archery, from the drawing of the bow and the release of the arrow to the mental projection which must accompany the arrow to the target. Thus the physical and mental coordination of archery, today as yesterday, returns full circle to haragei, the art of abdominal centralization without which, in Japan, coordination is considered inconceivable in theory and unattainable in practice.

from the book "Secrets of the Samurai: A Survey of the Martial Arts of Feudal Japan" by Oscar Ratti and Adele Westbrook (Castle Books, 1999)