Humans don’t give a lot of thought to the bottom side of their own tongues, but dogs make good use of both the topside and the underside of their lickers. They tend to use their tongues like little spoons; it curls backwards in order to scoop up the water and ladle it into the mouth.
Nina the boxer probably isn’t that used to drinking water from a fountain, but, after a couple of licks, she seems to decide that sticking her tongue over the stream of water and then scooping it up is easier than letting the water directly onto her tongue.
The Discovery Channel has a great slow motion video capturing the ladling action. Check it out here.