sit back vs unbend

sit back and unbend both are verbs.

Word	Noun	Adjective	Verb	Adverb
sit back	No	No	Yes	No
unbend	No	No	Yes	No

As verbs, unbend is a hypernym of sit back; that is, unbend is a word with a broader meaning than sit back:

Other hypernyms of sit back include decompress, loosen up, relax, slow down, unwind.

sit back (verb)	unbend (verb)
settle into a comfortable sitting position	release from mental strain, tension, or formality
be inactive or indifferent while something is happening	become less tense, rest, or take one's ease
	make less taut
	free from flexure
	unfasten, as a sail, from a spar or a stay
	straighten up or out; make straight