The Sodium Acetate is the anhydrous form of acetic acid, which is used as a buffer for acetic baths. The Sodium Acetate is commonly used together in toning baths and as a retarder in hydroquinone developers.
Sodium Acetate has a melting temperature of 51 C and below this temperature it prefers to be a solid. However, when Sodium Acetate is hydrated and melted it forms a supersaturated solution that will happily exist as a liquid below its melting point.
(Cooling the solution in a clean and texture free container is vital to prevent crystals forming and the solution solidifying prematurely). If this supersaturated liquid is then made to crystallise, the solution temperature rises to its melting temperature (around 51 C) immediately and solidifies.
This is where the name HOT ICE comes from and Sodium Acetate is used commercially for hand warmers, applying this very same principle.