This is what we'd call a hybrid craft if we've ever seen one. Using a rubber stamp on a cake does sound kind of strange (they usually go on paper, right?), but as long as these stamps are new and unused, they actually function really great for decorating on fondant.
With the perfect grooves for making indentations, you can stamp rubber stamps onto your cake to make pretty designs. Watch the video to get the how to. All the materials you need are a rolled out piece of fondant already applied on the cake and some rubber stamps.