The expansion (from heat therapy) and contraction (from cold therapy) acts like a pump. During the hot therapy treatment, the blood vessels expand, increasing circulation and the flow of healing oxygen and nutrients to the injury. During the cold therapy treatment, the blood vessels constrict, reducing circulation which allows the injured area to soak up the nutrient rich blood before it is pumped on. Hot and cold therapy work cohesively to help you recovery faster. The reusable pack is so easy to use! Pop the pack in the freezer for cold therapy, and pop it in the microwave for heat therapy – simple!