Page 119 - Barbecue Chicken Made Easy

Alas, only a few molecules will escape the can, and most go right out the top.
“I think Beer Can Chicken is a religion. We need a little separation of faith and science here.” Sterling Ball of BigPoppaSmokers.com
With a metal cylinder up its butt, warm air cannot enter the cavity from below and only the tiniest amount can enter from above. The can effectively prevents the chicken from cooking on the inside. All the heat must enter the meat from the outside. Because meat doesn't heat evenly, it progresses inward from the part in contact with warm air, and the outer parts are warmer than the inner parts. By the time the meat in contact with the can hits 160 to 165°F the outer layers are in the 180 to 190°F range. That may darken and crisp the skin a bit more, but it makes the outer layers of muscle drier.
Take away the can and heat will enter the cavity and warm the inside of the meat so heat is working its way to the center of the muscles from both sides. This way neither surface gets far overcooked. Remember, air cooks the outside of the meat, but the outside of the meat cooks the inside of the meat, not the air. The more meat the heat has to travel through, the more the outer layer gets overcooked. So cooking both sides insures the outer layers are not as hot and not as dry.