It's certainly not a new thing--these sorts of markings have been around for at least ten years as hobby-specific material, and a lot longer as dry transfer lettering and imaging from companies like Chartpak. I frankly don't see the appeal. Perhaps they're cheaper, or perhaps they adhere to surfaces that the waterslide stuff doesn't handle well, but you still need a smooth surface for optimum performance. Another possibility is that they don't require carrier film, which might make them a better choice for NMF finishes, except that you have no chance to reposition if you screw up, and they require significant pressure on the surface of the model, which some models and some NMF finishes might not cotton to. They also have a reputation for deteriorating on the sheet faster than waterslide decals, though YMMV, and they are also more prone to flake off the model over time.
Basically, the artwork comes off the bottom of the sheet, instead of the top for a waterslide decal, by pressing through the top of the sheet onto the surface to be marked, which (eventually) causes the dry transfer to part company with the sheet and attach to the model. The best tool is a dry transfer stylus. This allows you to apply pressure on the top of the sheet with as little or as much surface area as you need, and allows precision when dealing with the margins of the dry transfer. This is using the 'pointy-tongue' end of the tool. The other end of the tool is a rounded knob, which allows you to burnish the dry transfer through the sheet to ensure maximum lay-down and adhesion. Presumably they can thereafter be sealed with Future or other gloss coat and then a flat coat, which ought to minimize subsequent flaking, but again, if you're working with NMF, seal coats are a more complicated question.
Mark Schynert