The Broncos base 4-3 under:
Von Miller is the "S" and Dumervil is the "W"
He's on the line, but notice he's on the weak-side opposite Von Miller, in essentially the same position. Von will drop back into coverage more and Doom rushes more, but they're actually fairly similar roles the way the Broncos use them.
Edit: Doom is the capital W... the lower case w would be Wes Woodyard.