Kinda of stupid question but how does it hold on?

not stupid at all. It's got a sticky backing, just peel and stick. It's kinda like a decal, except stronger adhesion.

Here is an stupid question. What does it do?

no question is stupid. it kills vibrations and amplifies sound system. it also cuts out exterior noise and makes the area feel and sound solid.

ive always toyed with systems and ive never used dynamat. but my friend has he swears it makes a differance in his BMW 330i (2 - 12's W6) but idk i dont notice any but maybe cause i never heard it before. Im thinking about putting another mojo in my car is just the trunk and around the speakers good ? or is your just overkill :D

I'd say put it where ever you've got bass. If it's in the trunk, then you want to do as much as possible, including the decklid.

ok good to know :D

a lot of bass in just about ALL vehicles will rattle at the decklid, so it's probably the most important area to cover.