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'm interested in the C4s and thought I would hear them at RMAF, but they weren't in any of the rooms. It seems like the signature is the better value?

Ricred1 - Actually the C4 MKII is the best value but the signatures have a nicer finish. MKII/Signture/Platinum all sound identical. MKII came with a 5 yr warranty and the Sig's came with a 10 yr warranty but not sure about the Platinum.

Mechans - That tall pillar is the cabinet. They put drivers that are larger that the cabinet can accommodate. My C1's had a 6.9" driver in a cabinet that is only 5" wide.

Many Thanks everyone.

I have decided to go for the c4 signature and saved $5k and it was so nice
with my NUVISTA 800 for my Living Area.

Dreamauduio when you get the C4 Sig's, tell us how you like them?