If there is one thing I've learned in my two seasons of studying the Wade Phillips defense, it's that players have a remarkable ability to play better football when they don't have to think. Philosophically, the Texans' scheme operates on the concept that if players are too busy thinking about what they need to be doing, then they can't get busy doing what they need to be doing. To mitigate that problem, Wade Phillips makes his players think as little as possible. Read your guy, cover him if he runs a route, blow him up if he tries to run block, and do whatever you need to do to stay in your gap. See ball, get ball.
Phillips' scheme is pretty much just a few simple gap concepts and basic key reads (more on those later) with man coverage on the back end that is entirely tailored from top to bottom to fit the players he has to work with. Season to season, game to game, snap to snap, every single play call is completely dependent on who is on the field, what those players are good at, and what they aren't so good at. If a player like J.J. Watt is good enough to beat any guard in the league, Phillips would never be caught dead having him two-gap on that guard like most other 3-4 ends. He would rather make an outside linebacker two-gap, almost unheard of in the modern 3-4, than waste his best player on what is essentially a containment assignment. That kind of thinking is incredibly rare, and that is why this defense works.
Huge post, with a lot of pictures. I like pictures. Enjoyed reading, Hope you will too.