Solutions of a wide variety of inorganic salts used as an intermediary bath between washing and fixing will speed up the wash by improving its efficiency. Hypo clearing agents are a cocktail of the most effective salts, and are most useful with fibre-based papers, where fixing agents are most difficult to clear from the base material. Not really worth bothering with in the case of film and RC papers, which wash quickly in any case.

The old Kodak product is the favourite, supplied in powder and making 19 litres of working solution (3.8 litres of stock solution). All the others are liquid stock solutions.
Chemically quite benign, no special precautions needed.