The Logitech M570 trackball has a few underside features that allow it to be held in place from below. The battery cover plate must be removed to insert the unit into the holder. There is a carved hole to store the plate so it doesn't get lost. The unit fits snuggly into its place, to the point that I can even hold it upside down with out it falling out, but I wouldn't count on that. It stays put nicely in its cradle.