Why Did This Guy Screw a Piece of Wood to the Ceiling of His Car?

This guy is superstitious.  He and a buddy are in his car, and the friend asks why there's a piece of wood screwed to the car's ceiling. 

And he says, quote, "So whenever I say something, and I don't mean it . . . and I don't want to jinx myself . . . I can just knock on wood."