It's called pliability and is about training the muscles to remain relaxed under load.
He is remarkable in that most players are good for six years or so and he's working on over three times that with almost no serious injuries. He's been starting for fifteen years and has lead his team to more superbowls than anyone, ever.
He's published a book that details the method and some of the exercises and massage techniques involved, which is why he's promoting it of course, but the reviews are favorable.
In short, pliability training returns muscles to a less tense and more relaxed state. These long, soft and primed muscles better handle the stresses experienced by your body, shifting that stress away from your ligaments, tendons and joints.
He explains that an athlete who strength trains and doesn't focus on pliability will have tight, dense and stiff muscles that can't properly disperse forces, forcing your structural tissues to pick up the slack. If the force is too high, an injury will occur to those tissues. A pliable muscle acts almost like a shock absorber, helping to absorb and disperse those forces—one of the reasons he believes he can absorb hard hits.
Stack Magazine
Men's Health Review
Business Insider
Anybody own the book? Have thoughts?