Vague rule to me. I have never tapped on my marker with my putter, ever. If you have to do that then your marker is either in the way of another players putt, in which case it should be moved to either side anyway, or it could move based on a wind gust(?)
So you don't incur a penalty by tapping on the marker with your putter and it moves, even though you have already marked your ball and picked it up, hence finishing the actual act of marking your ball. But accidently moving it with your foot you do incur a penalty. To me in either case once you mark it and pick up the ball, the act is over. I have witnessed plenty of times when a player tapped on his marker with his putter and it flew up in the air and rested in another spot. They always moved it back, and I knew it wasn't a penalty, but I never agreed with it. But whether or not I agree with it doesn't matter.