There is the trigger of Hanare in Kyudo. What is the trigger is described in Kyudo Kyo - hon Dai - 2 kan (Kyudo Manual volume 2［Japanese edition only］).
At first, we should check the meaning of Japanse words: Hanare means "released", which is grammatically passive form of noun / verb, not "release", active form of noun / verb. Therefore, the string should be released, not an archer releases the string directly.
Then what is the trigger of Hanare?
Sakae Ukakami, Master of Heki - Ryu Insai, Hanshi - Ju - dan described the definition and trigger of Hanare, on 152 pages, in Kyudo Manual volume 2.
"Kai and Hanare should be clearly different status, and the trigger is needed to let Hanare happen. The definition of Hanare is to unlock the string from the groove of Yugake. However, if an archer releases the string intentionally by right hand, many bad things will happen. An archer has to make the trigger of hanare by left hand. More specifically, an archer has to make a last one push of side torque, with upside torque (so called Tunomi - no - Hataraki) . This is the trigger of Hanare"
For more information, detail is described in Kyudo Manual volume 2［Japanese edition only］), and please refer the articles below.
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(Cite) Kyudo Kyo - hon Dai - 2 kan (Kyudo Manual volume 2［Japanese edition only］)