Buddy Ryan was the toast of the NFL when he created the '46' defense for the Bears. (AP)

FOXBORO — Bill Belichick went into his teaching mode Friday and offered a PhD-worthy five-minute breakdown of one of the great defenses in history – the ’46′ Bears defense of Buddy Ryan, who starred as defensive coordinator of the Bears in the mid-80s before moving on to his job as head coach of the Eagles.

What was particularly fascinating was Belichick breaking down when the blitzing scheme worked – and when it didn’t. In short, the defense was designed to stop two-back offenses while one-back sets with two tight ends were very successful.

“A lot of the success that Buddy had with the 46 defense came in the ‘80s when there was a lot of two-back offense,” Belichick began. “It was one of the things that probably drove the two-back offense out. If you remember back in the ‘80s when Buddy was in Philadelphia, he had a lot of trouble with the Redskins and their one-back offense, a lot of trouble. There were a lot of mismatches of Art Monk and Gary Clark on the middle linebacker and stuff like that. I think the 46 was really originally built for two-back offenses, whether it be the red, brown, blue and the flat-back type offenses and eventually even the I-formation. I think it still has a lot of good application; a lot of teams use it in goal-line situations.”

In fact, those Redskins teams of the late-80s and early-90s won a pair of Super Bowls, going through Buddy Ryan’s Eagles in the NFC East on their way to beating the Broncos and Bills, respectively.

Belichick also got an up close and personal look at those Ryan defenses when the Giants had their wars with the Bears and, later, the Eagles in the NFC East.

“They either use a version of it like a 5-3 or cover the guards and the center and however you want to fit the rest of it, but that principle you see a lot in goal-line, short yardage situations,” Belichick said. “You see it and some teams have it as part of their two-back defensive package. As it has gone to one-back and it’s gotten more spread out, if you’re playing that, it kind of forces you defensively to be in a one-linebacker set. You lose that second linebacker and depending on where the back lines up and what coverage you’re playing, then there’s some issues with that. If you’re in a one linebacker defense and you move the back over and the linebacker moves over then you’re kind of out-leveraged to the back side. If you don’t move him over, then you’re kind of out-leveraged when the back releases and that kind of thing.

“There are some issues there that, I’m not saying you can’t do it, but you have to work them out. In a two-back set, I’d say it was probably a lot cleaner and it always gave you an extra blitzer that was hard for the offense. Even if they seven-man protected on play-action, there was always an eighth guy there somewhere. You didn’t have to bring all eight; if you just brought the right one and they didn’t have him or somebody would have to have two guys and that creates some problems. I think that’s what Buddy really, where the genius of that was; he had by formation a different combination and group of blitzes so depending on what formation you were in, then he ran a blitz that would attack that formation and then when you changed formations, then he would change blitzes. Now, plus the fact [he] had Dan Hampton, Richard Dent, Mike Singletary, [Otis] Wilson, [Wilbur] Marshall, that was a pretty good group there. You could have probably played a lot of things and that defense would have looked pretty good, especially when they put Hampton on the nose. That was pretty unblockable.”