What is the medical term meaning the act of rotating the arm so that the palm of the hand is turned backwards?

I am not sure what you mean by "turned backwards". If you are standing up or sitting upright in a chair, the act of turning the fore arm so that the palm of the hand is turned to the floor is PRONATION (Latin pronare, bent forwards). If standing up, the palm of the hand is turned to the sky or as if to scoop soup, this is SUPPINATION (Latin supinare, lay back). The same is true with the feet: turning your feet so that you can look at the upper side and outer edge of your foot foot and your small toe is pronation; turning your foot to see your soles is supination.