backstroke vs swimming stroke

backstroke and swimming stroke both are nouns.

Word	Noun	Adjective	Verb	Adverb
backstroke	Yes	No	Yes	No
swimming stroke	Yes	No	No	No

As nouns, swimming stroke is a hypernym of backstroke; that is, swimming stroke is a word with a broader meaning than backstroke:

backstroke: a swimming stroke that resembles the crawl except the swimmer lies on his or her back
swimming stroke: a method of moving the arms and legs to push against the water and propel the swimmer forward

backstroke (noun)	swimming stroke (noun)
a swimming stroke that resembles the crawl except the swimmer lies on his or her back	a method of moving the arms and legs to push against the water and propel the swimmer forward

backstroke (verb)	swimming stroke (verb)
swim on one's back