You can think of flinch as one quick bend away from something. A hand comes near a burn, a sound cracks behind you, and before any decision is made the shoulder pulls back, the eyes close, the head turns half an inch. Two things make the picture: it is small, and it is not chosen. Because nobody decides to do it, other people read a flinch as the truth leaking out. From that one movement the word goes inward, to the feeling that pulls back when nothing on the outside moves, and then turns into a test of character: the person who does not flinch is the one who stayed where most people would have bent.
From the word’s history
The source traces flinch to Middle French flenchir, meaning to bend, from a Germanic root meaning to bend or twist, attested in English since the 16th century. That bending is still the shape of the modern movement.
來源記載 flinch 來自中古法文 flenchir，意思是「彎」，再往上接到表示「彎、扭」的日耳曼語詞根，十六世紀起見於英文。今天那一下退縮的形狀，正好還是「彎一下」。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:722186996), etymology 1: from Middle French flenchir (to bend), from Frankish *hlankijan, Proto-Germanic *hlankijaną (to bend, twist, flex).
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