Why a bow may be broken when an archer shoots in bad way such as shooting with the nock off the string (Kara-Hazu) or terrible Mete-banare?
In an example of the bowling game, the floor of a bowling alley is reinforced to withstand the impact of the falling bowling ball, so it does not break, but if it were a normal floor made of thinner boards, the floor would break. In this way, damage to a bow occurs when an archer shoots in a bad way, not enough kinetic energy is transferred to the arrow, and the thin Japanese bow is subjected to a large amount of force that it cannot withstand. In other words, the large potential energy of drawing the bow during the shoot is not converted into kinetic energy that is transferred to the arrow, and this energy impacts the bow and the string. If the string is too tough and not to cut, the bow may be broken due to the impact.
To understand the causation of the impact on a bow in shooting, law of physics called the law of entropy is useful. Law of entropy explains that there are usable energy and unusable energy. In Kyudo, the potential elastic energy in Kai is converted into usable energy to fly an arrow and unusable energy to damage the bow when the arrow is released. Unusable energy is maximized when an archer shoots when the nock is off the string (Kara-hazu). This unusable energy does not disappear according to law of conservation of mechanical energy. If the string is not too durable (this is good for a bow), the string is cut and that would protect the bow. Therefore, the hard type sting is not good for bow when an archer shoots in bad way. That might happen even if an archer is advanced and well - trained, so we strongly recommend using soft type Kyudo strings such as Asa-Tsuru or soft type synthetic strings such as Senbon tsuru.
To avoid bow brokage, western archery is strengthened with more durable handle of hard wood such as walnuts or metal, and a wide limb which is dynamically efficient for the impact.
Meanwhile, in kyudo, bow brokage is avoided by the shooting method of releasing by YUNDE ( left hand ), that can most efficiently transfer potential elastic energy in kai to useful kinetic energy to fly an arrow. In Kyudo, the way to let the arrow fly is totally different from western archery. The string is locked by the Tsuru-Makura and Tru-michi (groove) in Yugake of Kata-Boushi (Kyudo glove) when Kyudo archer in drawing (Hiki-Wake). Kyudo archers gradually twist the bow in drawing process, and finally in Kai the string is released by the force of twisted bow. Twisting the bow by Yunde (left hand) is very difficult, and Teno-Uchi (how to grip the bow) is most important, and Kyudo archers need to practice for a long time in a right way.
In western archery, compound bow is the most dynamically efficient bow with a releaser. the bow equipment called releaser is to keep the string straight to minimize the vibration when shooting. In kyudo equipment, Tsuru-Makura and Tru-michi (groove) in Yugake of Kata-Boushi (Kyudo glove) work same as a releaser when an archer shoot by twisting the bow to unlock the string on the groove.
in the traditional Kyudo teaching, HANARE WO YUMI NI SIRASENUZO YOKI, which means "It is good not to let the bow and the string vibrate when releasing," In this way, the arrow flies sharply and straight, and hits the target, making it the most energy-efficient shot, and not to break the bow and even soft type strings.
（Cite）
・ Laws of Physics: Laws of Thermodynamics (MIT Open Course Ware)
・Mattew Archery official website
・Kinetic Energy (geeks for geeks)
・How Compound Bows Work（Outside）