You frequently hear that, but doesn't actually happen much.
I got around to trying "Last Man Standing" from Dear Mr. Fantasy,
by John Bannon. It's an in-the-hands Triumph effect using a concept called the Goodwin-Jennings display.
I followed the directions, riffled, turned the cards over, riffled again, and said to myself "That can't be right, I must have ACTUALLY mixed them 'face-up and face-down.'" And I went to unweave them...imagine my surprise. This is good.