The process the heat pack goes through is known as an exothermic reaction. The sodium acetate inside the heat pack freezes at 130º Fahrenheit but will stay in a liquid state at room temperature.
This is known as being super-cooled. With a click of the activator button, you cause a few of the molecules to become solid thus triggering a chain reaction. In the process of going from a liquid to a solid, the by-product is heat. These heat packs are fun to watch. Bending the metal disk starts the crystallization process, and you can see the crystallization proceed through the liquid.
What is happening here seems like magic, but the process can be understood if you think about water freezing. Water freezes at 32º F (0º C). If you were to stick a thermometer in a cup of water and put the cup in the freezer, you would find that the temperature of the water falls to 32º F and then hangs there until all of the water is completely frozen. Then the temperature of the solid water falls to the temperature of the freezer.
What if you could somehow super-cool the water?
That is, say that you could get the water’s temperature to 10o below the freezing point without it crystallizing into a solid — you can sometimes do this using a very clean glass and distilled water so there are no points for the water to begin crystallizing. In this condition, if you tap the glass the temperature of the water will jump up to 32º F (0º C), and the water will solidify quickly. HotShotz Heat Packs contain sodium acetate and water. Sodium acetate is very good at super-cooling. It freezes (solidifies) at 130º F (54º C), but will remain as a liquid at a much lower temperature and is very stable. Clicking the disk, however, forces a few molecules to flip to the solid state, and the rest of the liquid then rushes to solidify as well.
The temperature of the solidifying liquid jumps up to 130º F during this process. When you boil the solid, you melt it back to the liquid state. You have to completely melt every crystal, by the way, or the liquid will quickly re-solidify. You can repeat this cycle forever, theoretically, just as you can freeze and melt water as many times as you like.