You cannot knee a downed opponent in the head...you can knee him in the body. Silva cranked that knee squarely into Sonnen's body...it's the same strike Evans used to derail Tito Ortiz in their fight. Perfectly legal and very effective.
He kneed him right in the solar plexus. Silva knew exactly what he was doing.
Silva being taller then Sonnen, Sonnen sitting on his butt, how would Silva be able to drive his knee in Sonnen's chest?
Put your girlfriend to sit down on the floor and stand in front of her and see if your knee reaches her head or her solar. You would have to go down more on the other knee to be able to reach her solar.
While she's sitting down there anyway, get down on 1 knee and give her your flute: perfect height. Enjoy, you're welcome