Kūsankū is the Japanese rendering of Kwang Shang Fu (c. 1670-1762 CE), the ambassador from Qing dynasty China to Okinawa. Kūsankū, a kung-fu master, taught White Crane the Okinawans in the Kanemura/Naha area in 1756 CE. His student, Kanga Sakugawa (1733-1815 CE) 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.
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 occasional 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 a one-step processes are faster than two-step processes. This is especially important in kobudō, because two-handed weapons (e.g., staves, spears, swords, polearms, etc.) cannot perform Kūsankū movements. Many waza for defending against these weapons exploit this fact by using single-handed weapons wielded in pairs (e.g., [[Sai|sai], tonfa, [Kama|kama}, tanbō, etc.); one weapon is used to block/parry/check the opponent’s weapon, while simultaneously counterattacking with your other weapon before the opponent can react.
However, performing a Kūsankū movement will render you completely vulnerable. Since Kūsankū movements require extending both hands, you must spend a beat to retract your hands, briefly rendering yourself unable to block, counter, or disarm. There is no margin of error; Kūsankū movements must succeed, or be immediately followed by a perfectly-timed evasion to avoid the opponent’s counterattack. 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. Because timing is critical, Kūsankū movements are considered to be an advanced technique. 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 is achieved with no vulnerable gaps.
It may seem that Kūsankū movements neglect reciprocal action, but this is untrue. Reciprocal action largely serves as a mnemonic to remind students to tense the opposite side of their bodies in order to optimize snap. An advanced student should have enough control over their bodies to tense and relax in a way to make anything snap.