Don ßailey wrote:

Seriously, I wish someone could explain how snapping a little metal disk
causes the liquid to crystallize like that.

The sodium acetate contained in the bag is a "supercooled" liquid. That
is, at room temperatures it is below the temperature it would normally
change state back to a solid.

The click of that little metal disk provides just enough energy to
trigger "freezing" by raising the temperature of a molecule or two back
up to the freezing point and those first few crystals of frozen material
release their heat of fusion to a few more molecules and the process
continues until all of the contents are frozen. The heat that is
released is the heat that was put into the solid to melt it.

The melting point of sodium acetate is around 130 degrees F and that is
about the temperature you can expect the bag to reach.

Rick