The process used to heat it up is a health-hazard free reaction.
It is induced by the friction of the steel disc on sodium acetate, a saline solution used in the food industry.
Due to friction, crystallization is created, thereby releasing heat of 54°C for a period of time varying with the amount of liquid in the bag. The bigger the product, the longer the heat will last! After the heat emission, the liquid crystallizes. To reuse the bag, simply heat it up in water, to medium-high intensity, until no crystals remain. Let it cool down to room temperature and use it again!