what is the science behind it
Better reference than my rather flip comment about ramjet.
You see sean followed up with the same answer. But I've not seen any of these cardboard hovercraft. L
You know I didn't even look... Thanks man L
it works by compressing the air it "scoops" up and redistributing it to the edges of the vehicle to lift them off the surface and thereby produce a reduced friction environment. If you had a fan driving it, you'd have a ground effect airplane like the Germans devised prior to WWII (used for sea transport) and then carried into the Jet age by the Russians as the Ekta-plane.
When it is shoved forwards, I think the hovercraft scoops air in the front which supports it, so it slides along on a cushion of air. Neat.