It means with the talent, we become full DPS with tank gear, as sword-and-board. Without it, we're the same tanks we've always been. As I understand from twitter, can't switch between the two in combat.
It's like feral and guardian Druids. The style people want is the old one, where a Druid tank could go kitty for extra DPS when they weren't tanking. The style we'd get is basically a choice between guardian or feral, it's just a talent instead of a spec, if that makes sense.