Defense-adjusted value over average, it's Football Outsiders' secret formula for adjusting stats based on the teams played against in an attempt to make things more comparable, since no one plays the same schedule.
Speaking of how good the defense has been, I ran some numbers.
For the first 6 quarters, the Bills were allowing an average of 50 points per game and 438 yards per game.
For the 18 quarters since, the Bills have been allowing 14 points per game and 270 yards per game.
And even with the atrocious first 6 quarters, the Bills and the Jags are the only teams not to allow a 300-yard passing game.