"We have large veins under the tongue and the urine is absorbed directly into the blood stream. This is mostly for a message to the body. I don't have the scientific explanation for it but I know it help me a lot to get back to sleep at night."
It's called "sublingual" administration and it's pretty much as you said. There are some pretty big veins under there, plus the fact that the skin is thinner, so it's a lot easier to absorb some things and get them going through the bloodstream quicker.
Some hallucinogenic substances actually take effect quicker this way than if you were to ingest them normally, because the digestive juices can interfere and destroy certain compounds. (I'm not advocating any kind of "substance" use, I was just interested at one point and did a lot of reading on that stuff)
I've also heard comments about digestive juices destroy certain elements of urine, or making them less effective at least, so even when drinking urine normally it might be a good idea to let it sit under the tongue for a little bit with each sip .... think I'm gonna start doing that actually :)