The hot pack is a plastic bag filled with food grade sodium acetate and includes a small slightly concave metal disc. To activate the hot pack the disc is 'snapped' and the sodium acetate starts to crystallize and becomes hot. The hot pack is 'recharged' by boiling it in water for several minutes. 'Snapping' the disc starts the chemical reaction by initiating the crystallization, which is exothermic. It's a bit like scratching the inside wall of a flask containing a saturated solution with a rod; it gives a surface for the reaction to start on.