I hear a lot about rotation on the course in the short period of time that I’ve been playing. From a biomechanical point of view there’s something you should know: If you don’t fix your alignment you won’t fix your rotation.
Stand up and now slouch. Throw your hips forward, round your shoulders and look straight ahead. Now try to rotate. You didn’t rotate very far did you? Next try standing up with your best alignment. Now rotate. See the difference? If you don’t fix your alignment, you don’t fix your rotation. More next time on how to gain your best alignment.
You can also check out this YouTube video I did for rotation. Gaining rotation is about adding strength and endurance. Here’s a great exercise for both: