Snapping the disk triggers the crystallization process. The pouch is designed so that at room temperature the water contains many more molecules of sodium acetate than would normally dissolve at that temperature. This is called a supersaturated solution. The solution remains that way until it comes in contact with a seed crystal or some way of rapidly introducing energy into the solution which acts as a trigger for the start of crystallization. Snapping the metal disk inside the pouch delivers a sharp mechanical energy input to the solution that triggers the crystallization process.
Crystallization takes place so rapidly that the growth of crystals can easily be observed.
Heat is released during the precipitation that maintains the pouch temperature at about 54 degrees C for about 30 minutes. This makes the pouch ideal for a hand warmer. Furthermore, the pouch can be reused by reheating and dissolving the solid contents again.