Original: Hydrogen cyanide is produced when a solution containing a labile cyanide is acidified because HCN is a weak acid; alkaline solutions are safer to use because they do not evolve hydrogen cyanide.
Simplified: The poisons referred to are usually hydrogen cyanide (HCN), and the chemicals which are similar to it, like potassium cyanide (KCN), and sodium cyanide (NaCN). (Such substances are called derivatives of hydrogen cyanide).