Hops looses up to around 90% of its weight when dried. Rehydrating in the wort, it expands many times the size it was when you put it in...kinda' like a dry sponge acts when adding water to it...POOF!!!
Whirpooling moves the sediments, protiens and such to the center. When draining the wort, we try to keep most of the "crud" in the kettle.
Pick up one of the better homebrewing books(Papazian, Miller, or others). Combined with the great advise and experiences of the more advanced brewing gods of this forum, and you can learn a lot. They are always teaching me something new and improved.
