Kūsankū is the Japanese rendering of Kwang Shang Fu (c. 1670-1762 CE), the ambassador from Qing dynasty China to Okinawa. Kūsankū was a kung-fu master, and taught White Crane in Naha in 1756 CE. His student, Kanga Sakugawa (1733-1815 CE) later created the kata Kūsankū as a memorial to his teacher. This kata was later sampled and remixed to create the Pinan/Heian and Taikyoku kata series, which constitutes the core curriculum of most karate styles and many karate-derived martial arts. In Shōtōkan Karate, the kata Kūsankū is referred to by an alternate name, Kankūdai.
The kata Kūsankū is characterized by the repeated use of a simultaneous rising shutō block and a tate-shutō uchi. While Kūsankū is not part of the Goshin-Jutsu curriculum, we still make use of its core concept -- that defenses and counterattacks can be performed simultaneously. Within Goshin-Jutsu, any simultaneous block and counterattack is referred to as a “Kūsankū movement.”
Kūsankū movements are intrinsically faster than performing two individual movements. Whereas the standard block-counter combination works on two beats (i.e, ♫, “1-2”), a simultaneous “blockcounter” only takes one beat (e.g., ♪, “1”). Training to perform block-counter combinations faster and faster will mitigate, but not change the fact that one-step processes are intrinsically faster than two-step processes. This is especially important in kobudō, because two-handed weapons cannot perform Kūsankū movements. Many of the defenses against two-handed weapons (e.g., staves, spears, swords, polearms, etc.) work through exploiting this weakness. Single-handed weapons (e.g., sai, tonfa, kama, tanbō, etc.) are usually welded in pairs, so one weapon can block/parry/check the opponent’s weapon, while simultaneously counterattacking with the other weapon before the opponent can react.
However, there is trade-off -- Kūsankū movements create a "dead time" of complete vulnerability. Since Kūsankū movements require extending both hands, you must spend a beat to retract your hands, which briefly renders your unable to block, counter, or disarm. There is no margin of error; Kūsankū movements must succeed, or they must be immediately followed with a perfectly-timed evasion to avoid the opponent’s counterattack. Because their timing is so critical, Kūsankū movements are considered to be an advanced technique. Training to perform Kūsankū movements faster and faster will mitigate, but not change the fact that performing a technique and rechambering is a two-step process. This is why karateka usually prefer to use one hand at a time; by simultaneously rechambering one technique as the other is thrown, continuous fire can be achieved with no vulnerable gaps.
It may seem that Kūsankū movements neglect reciprocal action, but this is untrue. Reciprocal action is largely a mnemonic to remind students to tense the opposite sides of their bodies in order to optimize snap. An advanced student should have enough control over tensing and relaxing their bodies to make anything snap.