The phrase "in a reflex" is correct and usable in written English.
You can use this phrase when you are describing an immediate or instinctive action taken without conscious thought. For example, "He moved away in a reflex when he saw the snake."
In a reflex honed through thousands of innings behind the plate, Dwight Childs, a catcher for the St. Paul Saints of the independent American Association, pointed up the third-base line, appealing for a swinging strike call.
In a reflex honed _____ thousands of innings behind the plate, Dwight Childs, a catcher for the St. Paul Saints of the independent American Association, pointed up the third-base line, appealing for a swinging strike call.
"How do you do, son?" "In a reflex action, Russell takes it.
In a reflex mechanism called optokinetic nystagmus, the eyes pursue a moving scene to keep the image stationary on the retina.
A retired prostitute whom he meets on a bus refers to, perhaps in a reflex of professional flattery, "that burro's cock the devil gave you as a reward for cowardice and stinginess".
Satish had pursued Beth, but when he saw the little girl he hesitated, seemingly overcome, and dropped his arms to his sides, gathering his hands into his pajama top as though in a reflex of shame.
Once micturition is triggered via the PMC, supraspinal input subsides significantly and micturition proceeds in a reflex-like manner.
In contrast, innate olfactory behaviour lacks this integrative organization and rather is executed in a reflex-like way.
He doesn't believe in anything, a reflex to shield himself from pain -- a conceit, really.
Sighing is in fact a reflex.
3 The higher heart rate in the heat is suggested to occur in response to a reflex rise in skin blood flow 4 and an increase in relative exercise intensity, mediated by a hyperthermia-induced reduction in VO2max.
