Coffee Joulies are designed to improve the temperature profile of your hot beverage and improve your overall experience. To achieve this, they do two things:
Absorb heat to cool down coffee fast - To solve the problem of coffee being too hot at first, Joulies absorb heat from hot coffee and store it in something called 'Heat of Fusion' - more on this later. Only heat that is absorbed while the coffee is above 140F (60C) will be able to be released back into the coffee later. This is due to how PCM's work. How much heat is absorbed, and how fast, also depend completely on the conditions Joulies are used in. See the FAQ for more information on different containers.
Release heat to keep coffee in the perfect temperature range for longer - Once the coffee is below 140F any extra time that coffee would have been too hot is now extra time that you get to enjoy your coffee at the perfect temperature. The total amount of time before your coffee gets cold is dependent entirely on how well insulated of a container you use.
For more information on what is inside - What's inside a Coffee Joulie?
For more information on how Joulies work in different containers - How do Joulies work in different containers?