Hi Pat,
I'm not sure what this means either, but I'll give it a shot. There is a section of the back of the speaker that is open and a lot larger than a typical BR port or vent, and if you look in there you see the bottom half of the speaker is filled with folded felt and other acoustic stuffing, and the top half is basically open except for the rear of the tweeter assembly. There are 3 panels of different sizes that install over this opening in the back and tune the bass accordingly. The smallest panel gives the least amount of bass volume, and the largest, the most. It doesn't appear to alter the tuning frequency much, if at all, bu6t the level.
It's certainly an interesting design and there is something going on that is not anything like a BR speaker or an OB either.
And yes, I'm still loving them, especially since I put them up on some 3" thick maple cutting boards, and hung a thick, textured wall hanging between the speakers. Next step will be to install some short spikes in the cutting boards and then try some different feet under the speakers.
Hope that helps answer your question.
-- Jim