Sodium acetateis the chemical being used in the hot pack and it is a “super cooler” .This basically means it can be cooled below freezing point without it turning to a solid so it’s below its freezing point but at the same time it remains a liquid form. The thing about super coolers is they need something to disturb their molecular build up for them to turn into a solid and start the endothermic chemical reaction of heating up. Hence the metal disk that must be bent