On Friday, Rex Ryan made an interesting comment when he said that with how fast the new defense looks, it's possible the team could be looking at playing their base defense (a 3-4 look) even against a three wide receiver set. That's fairly unusual. When you see three receivers, a defense will often try to get an extra defensive back out there, usually at the expense of a linebacker. This will make a defense "small" and susceptible to a strong running game, with one less linebacker there to protect against the run.
By leaving the base set out there, it helps guard against the negative effects on the run defense. Presumably, this formation would be fairly rare against three receivers and would be highly situational. If not, it would be easily exploitable against pass-heavy offenses. It would leave Quinton Coples or Antwan Barnes against a slot receiver, which would be devastating if not purely situational. Another possibility would be Antonio Allen, who played a safety/linebacker hybrid known as the "spur" when he played at the University of South Carolina.
This would also be great against a spread offense, since it lets you cover the pass while also maintaining good run defense. But more than that, the reason this would be so good, if actually possible, is it is a perfect defense to Bill Belichick's (and Chip Kelly) hurry-up offense. Usually, the team on defense will be forced into a bad matchup and it will be exploited the entire length of the field with no time to substitute players. If the New York Jets can operate out of their base defense, it will be harder to get them out of position because of the versatility as listed above.
It's also noteworthy because the Jets and the New England Patriots have built their respective teams around stopping each other (for obvious reasons... win the division, and you make the playoffs). The Jets build up their cornerbacks to stop the Patriots receivers (Antonio Cromartie and Kyle Wilson), so the Patriots get Rob Gronkowski and Aaron Hernandez. The Jets can't cover the tight ends, so they decide to apply pressure to Tom Brady himself and draft Muhammad Wilkerson, Coples, and Sheldon Richardson. The Patriots then add the hurry up offense, which conveniently makes creating pressure more difficult. As a result, the Jets look to increase speed on defense and allow the versatility which stunts the hurry up. It's surprisingly logical flow and interesting back-and-forth.
This, of course, is predicated on players that are able to transition between the two. That's where Ryan's comments come in to play. If the base defense can make the transition Ryan seems to believe they're capable of, opposing teams are going to be in trouble.