Like humans, some dogs are better swimmers than others. And like all swimmers, they will eventually tire. Even a good swimmer might be disoriented from the shock of falling in the water. Most pet life jackets are designed to keep the animal in a horizontal swimming position. The UnderDOG is different. By moving 90% of the effective buoyancy under the chest and neck, the vest is designed to allow a more natural swim angle with the hind legs lower than the front.