Heat Tech t-shirts, sweaters!

Hey guys, anyone heard of Heat Tech from Uni Glo? They were suggested to me by my flat mate, they come in polo necks, t shirts and sweat shirts. I ended up getting a few for riding after all its 2 for £15 at the moment. Don’t know the tech behind it, but its basically a material that heats up from rubbing against your body using the cold sweat or something and it stays cool in the summer.