expression, godan verb (-ru)
clapping wooden clappers to signal the start or end of a performance
Rare term from traditional Japanese theater (kabuki, bunraku). Refers to the sound of wooden clappers (柝, ki) used to mark the beginning or end of a performance.
歌舞伎では、柝が入ると幕が開く。
In kabuki, when the wooden clappers sound, the curtain opens.
Refers to the wooden clappers themselves, while 柝が入る describes the action of clapping them as a signal.
Compound of 柝 (ki, 'wooden clappers') + が + 入る (hairu, 'to enter'), literally 'the clappers enter'. The exact historical derivation is uncertain, but it is a set phrase from traditional theater.