When you are trying to remove inflammation from an area I.e. twisted ankle, you need to cool it as much as possible. That reduces the spread of inflammation and this includes reducing blood flow. Do that for a day or two after an acute injury or surgery, maybe longer to prevent inflammation. Once the tide of inflammation is under control, you can use heat to get that blood flow back and to allow removal of breakdown products and to bring in more blood to heal. There are situations where you will alternate between hot and cold but usually not with an acute injury.