Although this match was not his best (the drop to 170 took its toll), it was interesting to watch his game - going for the takedown, seemingly not worried by guillotine attempts, easily mounting another fighter with a very good ground game.
Shields calls this American Jiu Jitsu, based on his background as both a collegiate wrestler and a BJJ blackbelt. It has an emphasis on the high pace and pressure of wrestling with the skill and technique of jiu-jitsu. Or another way to put it is an emphasis on takedowns and top-game from wrestling and subs and sweeps from BJJ.
Calling his style American Jiu Jitsu has some people cheering and other people seriously bent. The cheerleading side says it is great that he is bringing more takedowns and top pressure into jiu jitsu, the naysayers claim he isn't really doing anything new and therefore doesn't deserve his own style name it is just BJJ.
My take is that calling what he does American Jiu Jitsu is as legitimate as any other style name in martial arts. Very rarely can a "style" lay claim to inventing something new. What is different between "styles" is where they choose to put an emphasis or use their training time. Such as TKD emphasizing kicks vs. many other styles of karate or Judo emphasizing throws vs. BJJ emphasizing ground work.
A good Judo guy is going to know plenty of ne-waza but his throws are going to be better because that is his emphasis - a good BJJ player is going to know throws but his ground game is going to be better because that is where he puts his time.
AJJ is saying that they spend more of their emphasis/training time on wrestling/takedowns than a typical BJJ school (and I don't think they really do any gi work at all). It is all about where you spend the limited amount of training time you have. A style/name is just a convenient label that let's people categorize things and let's the label's creator/owner differentiate himself and market more effectively.
Visually it might look like this (don't critique the artwork I took like 5 minutes ;-)):
There are only so many hours in the day and you have to divide your time up somehow.
