retroflex vs flex

retroflex is not a noun while flex is a noun.

Word	Noun	Adjective	Verb	Adverb
retroflex	No	Yes	Yes	No
flex	Yes	No	Yes	No

As verbs, flex is a hypernym of retroflex; that is, flex is a word with a broader meaning than retroflex:

Other hypernyms of retroflex include bend.

retroflex (noun)	flex (noun)
	the act of flexing

retroflex (adjective)	flex (adjective)
bent or curved backward
pronounced with the tip of the tongue turned back toward the hard palate

retroflex (verb)	flex (verb)
articulate (a consonant) with the tongue curled back against the palate	cause (an object) to assume a crooked or angular form
bend or turn backward	bend a joint
	form a curve
	contract
	exhibit the strength of