It's a 3 part foam system what guides the Rear and Front waves where they are intended to go, as well as some rear vibration support on the door skin.
The part I really liked about this kit is that the front baffle is deep enough to be trimmed to fit, but with this application, the speaker was about 1" from the door panel and this made it fit snug and tight with no sound waves bouncing around inside the door panel.
Although, Hybrid makes and amazing speaker by itself, I could tell the difference this kit made....customer had a smile from ear to ear.....mission accomplished!