The flippant change is the perfect example of a knack in magic. do it six million times, then you can do it. we can give you tips, but that wont do the trick, as you know.
just keep your movements smooth.. it's very hard.
Make sure you're hand is about 40 degrees tilted clockwise (anti-clockwise if you are left handed). Quite a big jerk downwards is needed in the beginning (as I remember when I was practising) but after some practice, a very small jerk is needed.