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You can't let the opponent see that you've flinched.
Literal
Flinched [nominalizer-の] [object-を] opponent [dative-に] be-noticed-[passive-れる]-[must-not-てはいけない].
A tactical observation. 怯む ('to flinch, to be cowed, to waver') is used for momentary loss of nerve. 気取られる is the passive of 気取る (keidoru, 'to sense, to pick up on'; distinct from the homographic かだる, 'to put on airs'). ~てはいけない ('must not X'). A tense-filled moment in what sounds like a confrontation.