Dynaudio c4 signature & c4 platinum difference

Please forgive me in advance about the Dyne Audio C speakers. What role does the large almost slab looking pillar open to the drivers back action? It looks like the drivers are put the baffle which is then attached to the big structure, but open and communicating with the pillar? I am sure it was designed to look like that but isn't.
Can some one explain how the speaker works, e.g. the tweeters do not communicate with the large structure, etc.

I still wonder why the baffle is wider than the cabinet. What is gained using this approach, beside mounting larger drivers ? I still have trouble understanding the benefit of such an arrangement.