Steve at the Firearms Blog has all the specs and some pretty pictures of the system, so go check it out. It holds sixteen rounds of 12 gauge in a magazine which the shooter has to rotate every four shots:
Makes me wonder how hard it would be to create a magazine system similar to this, but where you rotate it after insertion to wind a spring, and the rotation happens automatically during firing. Of course, that might be a little bit too much like this for comfort of the manufacturer.
The problem with semi-auto tactical shotguns is that the magazine needs to be huge to have a reasonable capacity. This is an interesting solution.