I have a very simple description of such things. From you current position till the time you have moved to the desired position, completed your back and front swings, with or without a loop, and contacted the ball - call that interval of time T and the distance you are moving D.
Your position in the court and the racket position wrt your body both need to change from your current configuration to the final configuration within T and D.
Make the movement as smoothly continuous as possible over T and D. That is all.
There may be jerky adjustments needed in some cases (like you are running wide and then want to flick the ball in with a quick swing at the last moment), but overall it should be smooth to prevent injury.
Keep in mind that not all pros do this. Some like Roddick prefer to get to a far ball first and then start racket takeback and finish it quickly, others start taking it back smoothly earlier on the run in such a way that the forward swing finally meets the ball. Some like DP and Soderling might prefer to wait for the ball with the racket back much earlier than others, hoping for a huge and fast swing into the ball.