This is a heavy beanbag that contours to fit snugly on your dashboard. I suppose it works because of its weight. It doesn't move around at all.

I'm not certain how our GPS was assembled but it does have a suction cup attached now. Our GPS just suctions directly onto the flat surface of the beanbag to hold it securely.

I got this informaiton from another site: It answers your question.
The Nav-Mat is the most convenient mounting accessory for your portable GPS today. Using the physics of weight distribution along with high tech anti-skid materials, it creates a solid mounting base while turning your windshield mounted GPS into a dash mounted GPS. The Nav-Mat conforms to the contours of your dashboard, allowing it to work on uneven or textured surfaces while the anti-skid underside keeps the device from sliding. Designed to work with devices up to several pounds using the stock windshield suction mount sold with most popular GPS units, the Nav-Mat requires no tools for installation and can easily be moved from vehicle to vehicle or put out of sight when leaving the vehicle. Also works well with other Bracketron Windshield Mounts for mounting devices such as Satellite Radio, Cell Phones and MP3 Players.
Works with any brand that uses a suction cup mount (Garmin, TomTom, Magellan, etc...)