Whirlpooling

Whirpooling employs 2 methods of seperating the trub from the wort. The first one is sedimentation, which means the trub will sink to the bottom when left alone. The second one is centrifugal force which forces the trub into the center of the pot. If both methods are used, the trub will be collected in a nice trub-cone in the center of the pot. This is the main trub seperation technique that is used in commercial breweries before the wort is chilled. In a home brewing set-up, whirlpooling can be used before or after the wort is chilled. The latter also allows for the partial removal of cold break before the wort is transfered to the fermenter.

Just sedimentation would work to. But since the trub would be evenly spread on the bottom of the pot, you cannot siphon as low as you can if the trub is collected in a cone.