noun
winning technique (sumo); clincher; decisive move
Primarily a sumo term for the technique with which a wrestler wins a bout. By extension, used figuratively for a decisive move or clincher in other competitive or argumentative contexts.
横綱は寄り切りを決まり手にして勝った。
The yokozuna won with a yorikiri as his winning technique.
議論の決まり手は彼女の一言だった。
The clincher in the argument was her single remark.
決め手 is a more general term for a decisive factor or clincher, not limited to sumo. 決まり手 is specifically the official winning technique in sumo, though it can be used figuratively.
Compound of 決まり (kimari, 'decision, settlement') + 手 (te, 'hand, technique'). The term originated in sumo to denote the technique that decides a match.