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underhand grip (e.g. in gymnastics, etc.)
cross-handed grip (e.g. in golf, etc.); using one's hands backwards from their conventional positioning
reverse grip (e.g. of a sword)
turning the tables (on an opponent)
reverse arm-lock (in judo); joint lock
逆手に持ったナイフで木を削った。
I whittled the wood with a knife held underhand.
相手の逆手を取って反撃した。
I turned the opponent's move against them and countered.
鉄棒を逆手で握ると力の入り方が変わる。
Gripping the bar underhand changes how you apply force.