There are two small coolant lines that run to the top of the aluminum intake manifold to prevent throttle body icing. Some people remove these from the manifold and connect them together to bypass the manifold. The theory is that the cooler manifold will allow a denser charge and more power. It is certainly directionally correct for performance but the gains are tiny.
This will only work on the aluminum manifold trucks that have the coolant lines. Since it can result in a stuck throttle plate, it is not the best idea that ever came along.