But hey, their backstroke I'm tryna perfect

from Kanye West (Ft. Charlie Wilson) – Bound 2 Lyrics on Rap Genius

Meaning

Ye is “backstroking” through women, which combines senses of both “swimming” in women and “stroking” (a sexual thrust) from the back.
http://www.youtube.com/watch?v=HH78mglJcnw

The backstroke also makes a double entendre out of “rep” in the previous line, since each stroke is a rep in swimming terminology

Kanye says he wants to perfect their backstroke, not his own, implying these girls needed the practice/guidance from him.

You know how Yeezy teaches ‘em.

To help improve the meaning of these lyrics, visit “Bound 2” by Kanye West (Ft. Charlie Wilson) Lyrics and leave a comment on the lyrics box