Whirlfloc is basically dried seaweed (Carragheen or Irish Moss) so maybe it swells up a bit as well while it is doing its coagulating job.
Whirlpooling is just stirring the pot so the wort is wooshing around in a circle, that causes the trub to collect in a little hill in the middle of the kettle away from tap or syphoning point. Try it in a large mug with tea leaves and you'll get the idea.
In full size breweries it's a big deal because they have to invest in pumps and paddles and wheels etc but for HB it's just a bloody big spoon so no problems. Polyclar is a PITA because you have to stir it for 20 mins but works great, especially if you have also cleared the beer with gelatine first.
For my imminent AG brews I intend to use all three, whirlfloc, gelatine, then Polyclar as I won't be filtering and intend to go into comps next year so need clarity.
Hey Reviled, Polyclar is really cheap, recommend you give it a go.
Cheers
Edit: If Polyclar is used strictly to directions, the effect is usually 'instant' - I usually let the beer warm up to room temp overnight and bottle immediately. The tiny amount of polyclar (PPV) that settles to the bottom of bottle or keg is no problem. It's a widely used chemical and totally neutral. If you have taken an Ibuprofen painkiller recently you'll have PPV going through you right now, and it's probably in your hair conditioner as well if you use one.