I think it goes like this. A defensive player gets a turnover in the field of play, but for whatever reason carries the ball into his own end zone where he is tackled.
The only other one I know of was when Texas scored one against TAMU in 2004. I saw that one too on television.
Never had heard of it and I thought i knew every rule. Especially the scoring ones. I wonder how they came up with that? If he runs it back its 6 points right? why wouldnt it be -2?