Why did Sir Garwain flinch when the ax was swung?
Concerning Sir Gawain's flinch in Sir Gawain and the Green Knight, wouldn't you? A big ax is being brought down toward his neck by a giant green man with the intent of chopping his head off. It shouldn't surprise you that he flinches. Flinching is a natural human reaction in this situation.
And that's the point. The poem is somewhat of a breakthrough in literature. It's a romance work that doesn't depict a hero that is perfect. The work is a small step toward Shakespeare and modern literature, which feature flawed characters. Gawain is a forerunner of the anti-hero, albeit far removed, very far removed.
Gawain is still, overall, noble and brave and chivalrous. But he is not perfect. He fails to give the magic girdle to his host as he has agreed to do and, yes, he flinches when facing the ax. He is more human than most Authurial-type romance heroes. By our standards there's nothing wrong with flinching when facing a blow. But for a medieval romance hero, this was a big step.