like the Native American Pocahontas may have participated in a tribal rain dance in hopes that rain would fall from the sky, the female subject in question is dancing and hoping for rain (but in this case in the form of paper money dropped from above).
the flow (or smooth rapping ability) is "retarded" like the idiot-savant character in the movie Rain Man. however, used here this doesn't mean slow - it means really good. "retarded" may come from crazy/mad/sick which can all be positive adjectives.
watch how i hold my money in one hand, lick a finger on the other hand and slide it toward me on the top of my stack of money to get the top bill (buck, dollar, but probably a bill larger than $1 in denomination) to peel back toward me as i pay for things