It is a folded pouch, the stuff is between the folds and comes into contact with the liquid in the pouch to start the reaction. They are pretty cool, but very scary. Be extremely careful when you finish that beer and open that pig!!!! My advice, get it very cold before you try to open it to lower the pressure with temperature, and have a ice pick ready to poke the bag. Never completely remove any screw, and back them all off in increments until you see the bag. It is amazing how large the bag is when you remove it.
Really cool product, and I am glad it finally started working for you. It will go through spurts of more and less pressure with the bag popping right before you start to get pressure again.
I am absolutely scared of these things though until you get that bag popped though, be careful!!!