This demonstration is a good example of an autocatalytic reaction.
Many reactions are speeded up by other substances, a phenomenon called catalysis. In some cases, though, the catalyst is one of the products of the reaction so once the reaction gets under way, the products catalyse its continuation and even sometimes speed it up.
In this rather attractive example, a small volume of acid is added to the top of the cylinder to initiate the autocatalytic reaction. A yellow layer is formed. Within a few minutes the yellow layer moves down the column as the autocatalysis effect takes place.
Autocatalysis – Risk Assessment