If there is one string that seems a bit duller or quiter than the others you can raise its pole peice
Its very similar to how changing the pickups height affects the tone, like feline said - more treble and sensitivity.... But the polepieces let you do it for each string individually. so first off set the pickup height - then do the polepieces individually
If you take the idea to the extreme you end up with something like the q-tuner pickups that have loads of polepieces to adjust for tonal shaping purposes
http://www.q-tuner.com/adjust.shtmlGreat idea but since playing about with just 6 polepieces can take a lot of trial and error the q-tuner seems a little overkill
(PS - i woudlnt normally link to another pickup maker here but i figure q-tuner and BKP are distinct enough products for it to be ok.... somebody remove the link if its a problem)