It's a box with a messed up port thing inside it that tricks the sub into bieng two subs pretty much, so you get twice the performnce out of 1 sub. Make sense?I think SREDISH or RNBRAD could really expand upon this and how a folded horn works. I'll sell these for $75 plus the shipping for both 10" and 2" boxes. These retailed for $150 at the local precise sound shop.