the technique is simple, to cook something in liquid slowly. normally it is intended for a tender protein product, such as poultry, fish or even eggs. the trick is to maintain a slow even heat and cook just to point of 'doneness' the poaching liquid should never boil. the most technical form of poaching is sous vide, where you maintain an even temperature and constant temperature by circulating the poaching liquid with pump and enclose the item to be poached in a special plastic wrapper.
to understand why poaching is great, you have to understand the science of protein and i will explain it briefly. protein is composed of strands that contract when heated, if you heat it too quickly the protein becomes tough, if you heat it longer at a slower temperature those strands relax and become more tender. a poached piece of fish or even a chicken breast will be tender and moist without being overcooked and flavorless. the choice of poaching liquid can even add more flavor. i will now explain how to poach a chicken breast simply and without overcooking. the texture of a properly(fully cooked) poached chicken breast may be unfamiliar since most people are used to overcooked chicken.
the key in my book to poaching is the slow heating and not so much the rapid heating. select a standard package of chicken breast(2-3 half breasts) of maybe a pound or more. purchase a 2 quart package of low sodium chicken stock(or use homemade). place the chicken breasts in a kettle deep enough to hold them and add chicken stock to cover. set a low fire under the vessel and wait it out for maybe 20 minutes. changes will occur and at no time do you wish for the stock to boil rapidly. when the internal temperature of the chicken breast reaches 165 F it's done. i had to say that, i actually have been known to eat a clean, fresh chicken breast at 145 F but i cannot legally recommend such a practice. now serve the no longer pink, but very tender chicken breast in a salad or chopped in a pasta dish; or just keep it your fridge for a snack with a little honey mustard.