They expand and stay in the body, so all the kinetic energy favored for stopping power is maintained.
I wouldn't reccomend cutting Xs in your ammunition, as it could lodge in the barrel if you cut it too much, and never use them in anything other than a revolver. You'll just get lots of jams.
Here's a shot from youtube, it's low res though, because I wasn't going to fuck around with HD for an hour to grab one shot:
