From the P-G, defensive wizard Dick LeBeau apparently introduced a new defense during minicamps that the writer facetiously labels the "99 defense," and might be more accurately called a 2-5.
In this new formation, right DE Brett Keisel (the 99 of the story) would be turned into a fifth linebacker, in the same sort of role that Troy Polamalu has played. Instead of rushing the QB on most downs as he does now, he'd be used more often in pass coverage, as well as providing run support as normal.
In a way, it's a natural evolution of both the 3-4 defense that the Steelers have used for the past 15 or so years (one of its primary advantages is that, even in a normal pass-rush, the offense doesn't know who the fourth or fifth rushers are going to be) and the zone-blitz that LeBeau invented (which drops a lineman into coverage and blitzes a defensive back or linebacker, confusing the hell out of the offensive line assignments).
Just as offenses for the past few seasons have had to check and see where Polamalu is before each play, they'll now have to do the same with Keisel, who is less athletic than Polamalu but larger and stronger at 6'5 and 285. And even if he's in a three-point stance alongside Casey Hampton or standing with the linebackers, the opposing team still doesn't necessarily know what he'll be doing. He could blitz from a two-point stance or jump back from the line during any given play.
So now instead of having to look for the fourth rusher on every down, offenses have to search for the third and fourth. I like this development a lot, and I'm interested in how our Timmons and Woodley will fit in.