But hey, their backstroke I'm tryna perfect
The backstroke also makes a double entendre out of “rep” in the previous line, since each stroke is a rep in swimming terminology
“Backstroke” refers to positions involving the woman lying on her back, i.e. while having sex (“flat-backer” being a common insult implying a woman is promiscuous). Kanye says he wants to perfect their backstroke, not his own, implying these girls needed the practice/guidance from him.
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