Whirlpooling is a common method used in commercial breweries to separate hop pellets and trub from wort after the wort boil. Essentially the wort is pumped into the whirlpool vessel at rapid velocity, usually about 15 feet per second, to cause the wort to start spinning like a whirlpool. Sometimes the kettle doubles as the whirlpool vessel and the wort is recirculated to the kettle.
In commercial breweries this pumping process lasts for 10 to 20 minutes. Then the wort is allowed to stand for an additional 20 minutes or so to allow the hops and trub to form a compact trub/hop pile in the center of the vessel. The wort is easily separated from the pile by pumping it out of an outlet located on the side of the vessel. Although whirlpooling was originally developed to separate pelletized hops from wort, breweries that use whole hops, such as Anheuser-Busch, use whirlpools to remove trub after their whole hops have been strained from the wort.
Homebrewers can use whirlpools just like the big guys. Instead of pumping the wort after boiling, use a big spoon to stir persistently in a circular motion in one direction. This will generate a mini-whirlpool. Stir the wort at a good clip for one to two minutes and allow it to rest for 10 to 20 minutes before draining the wort into your wort cooler. Watch as the wort level gets low. If you really want to separate as much hops and trub as possible, stop the flow when the trub pile starts to suck into your wort chiller.
The whirlpool is a simple device, but there are many variations on its basic design used in commercial breweries. The most common design is a flat-bottomed vessel with a slight pitch toward the drain. This gives a great trub pile but also allows the trub to slide toward the outlet if the pile is loose. Some designs have a low point in the center and an outlet drain that is a little higher than the low point. This prevents trub from leaving with the wort. Your keg/kettle will behave like this system.
Whirlpool trivia: Some tea drinkers who use loose tea and no tea ball separate the tea leaves by stirring their tea after the leaves sink to the bottom of the cup to cause the leaves to migrate to the center of the cup. Albert Einstein thought this phenomenon was pretty nifty. He developed a general explanation, called the Teacup Effect, of why pressure changes cause solids to migrate to the center instead of to the perimeter, where it seems like they should migrate. Molson Breweries in Canada is credited with first using the whirlpool in commercial brewing.