the hop blocker works by drawign wort off at different points along the water column.
The reason it works is so stupid and simple that it's pure elegance (save for the part about having to jam crap in your clean wort and manipulate the gizmo). It is so elegant and simple in concept that I started kicking myself the moment I saw it because I didn't think of it first.
Fast flowing fluid carries crap along with it. Fluid in a pot flows out from a port in the bottom fastest when the water column is high. That's it~!! That's the whole concept in a nutshell.
I really don't like how Blichmann executed on the concept, but that's another story.
At any rate the thing works because of the weight on the water column. When there's a lot of wort in the BK there's a lot of force pushing wort through the dip tube. This high force causes a strong rush of fluid which picks up all the trub, hop debris, and crap from the cold break and carries it to the dip tube.
The hop blocker initially draws wort off from a higher point, thus accomplishing two things: (1) it is drawing wort a few inches above the trub and crap where it will have to fight gravity to climb the water column to get out and (2) it takes wort out higher up where the force of the rush of fluid is lesser. So, you get less trub.
Throttling on the valve is one way to reduce the rush of wort. Depending on how well settled the cold break is you may want to do this. You could also let the wort rest for a few hours to let the cold break congeal. I do this. I have also just transferred everything to the conical, let the trub settle out there ,and dumped the break and trub from the conical later on.
I haven't ever quite gotten my head around the whole "whirlpool" thing.
Never works for me.
No matter what I do, the trub ends up dispersed in an even self leveling layer across the bottom of my BK. So if I want to eliminate as much trub as possible, I just put a little block under one edge of the pot (tilting it) so the trub flows to the lowest point, leaving one part of the pot largely trub free (where the pick up tube is).