also: みじかで
noun
silent clap
A Shinto ritual gesture: clapping without making sound, performed at funerals or certain Ise Grand Shrine rites.
神葬祭では、忍び手を打つことがある。
At Shinto funerals, a silent clap (shinobide) is sometimes performed.
伊勢神宮の特別な儀式で、参列者は忍び手をした。
At a special ceremony at Ise Grand Shrine, the attendees performed the silent clap.
柏手 is the standard audible clap in Shinto worship; 忍び手 is its silent counterpart used in solemn or funerary contexts.
Compound of 忍び (shinobi, 'stealth') + 手 (te, 'hand'), referring to a hand-clap performed silently. The reading みじかで for 短手 is rare and likely a direct kun'yomi reading of the characters meaning 'short hand'.