Stirring mixtures “over ice” is an extremely useful technique. It is used when working with gelatin in order to speed up the jelling process and at the same time give you control over it so it doesn’t jell too much (which might happen if you leave the gelatin mixture in the refrigerator). You can also use it any time you want to cool down a hot mixture quickly.
To cool things over ice, place the saucepan or a metal or glass bowl containing the mixture in question in a larger bowl filled with ice and a little water. The water is needed in order to completely surround the saucepan or bowl with cold. If you are cooling a hot mixture or thickening a gelatin mixture, you must stir it, which both speeds up the cooling process and, in the case of gelatin, prevents uneven jelling.