Here's a video to show how I start teaching thumb position
It's really important to think of any position you are playing in, whether it is 2nd or 5th or a thumb position in two different ways. When you are thinking of the notes that you can play in that position, don't let that make you feel that your hand or arm has to stay put and the fingers and thumb just have to lift or drop. If you are playing slowly, each finger moves to a place of balance taking with it any other appendage that needs to go along for balance. Playing really fast in a position does call for less movement but that is something that usually happens naturally. It's important to know what the notes are that you are playing in any given position but don't let that knowledge lock you into a tight place.
