what the heck is dynomat?

I went to best buy to buy my new pioneer front and rear speakers. The first thing the salesman tried to sell me is the "extended warrenty" for 40 bucks, hah, not like im going to blow the speakers out without an amp anyways. The second thing he tried to sell me was something called Dynomat? I guess its some sort of foam or something that redirects sound better or something. The stuff was 20 bucks, and since I didnt know what it was, I didnt buy it. Does it really make that much of an improvement?