When obtaining a cherry, your character splits into two. Now, you can have two Marios running around in tandem, jumping and stomping in sync. If you grab another cherry, your two Toads will become three. Grab another, and your three Peaches will become four, etc. Best of all, you can combine this power-up with any other. At one point in the demo, I had five fire-powered Peaches laying devastation to a pack of enemies. It was wonderful.
You can't control doppelgangers separately from your main hero, meaning they will eventually fall out of sync with each other. That poses some interesting platforming challenges, as you try to get a horde of characters into a warp pipe. You'll start taking advantage of walls, or periods of time when your "main" hero is unaffected by controller input, in an attempt to reorganize your group. There's benefit to maintaining a large crowd, too. Not only is it incredibly empowering to see an army of clones on the screen, there are certain areas that are blocked off unless you have a certain number of doppelgangers. Because each of your clones will disappear in one hit, there's an increased challenge in dealing with the growing horde of characters.