Pole pieces act like microphones in front of a speaker, to an extent. Move 'em closer and the string is louder. The slugs coil is non-adjustable relative to the pickup bobbin, so the only adjustment you get is via the pickup's height adjustment screws. The screws can be adjusted relative to the pickup and, unless screwed in until they are flush with the top of the coil bobbin, are starting off a little closer to the strings.
Adjustment gets you a number of things:
- Adjust string-to-string balance
- Adjust coil-to-coil balance within the humbucker
- Adjust the tonal balance of the pickup overall (when combined with pickup height adjustment)
String balance can be much improved by screwing the fat E pole in a turn, the D pole out a turn, the G pole in a turn (or out 2 turns if wound G), and the skinny E pole out a turn (as a starting point for fine-tuning string balance). It's a small difference, but it helps all the notes be heard within a chord.
You can add oomph to the screws coil by turning them all out a turn or two. The screws coil is often a little bit quieter than the studs coil, due to the smaller mass of the screws and the inefficiency caused by the screws sticking out of the bottom of the pickup and diverting some of the magnetic field around away from the strings. The studs are a bit fatter than the screws (which lets them load the coil a bit more and raises that coil's inductance a bit) and they don't stick out the bottom of the pickup, so the slugs coil is a bit more efficient at picking up string vibrations. That said, magnetism follows the inverse square law (double the distance, and you cut the magnetism by a factor of 4; triple the distance and you cut the magnetism by a factor of 9, i.e.!), so at the small distances involved in pickup adjustments, a small change in the proximity of the pole piece to the string can change the string's volume significantly.
If you screw the pole pieces out several turns, and back the pickup off, you can get a considerably crisper sound out of a pickup, as the slugs coil will now be considerably farther away, and the screws coil will be louder. Want a less wooly neck 'bucker tone? This is one way to get there. If the pickup has 3 or 4 mounting screws, and you can tip the whole pickup, you can do the same thing. The more difference in the coils' proximity to the string you can get, the more like a single-coil it will sound. The Gretsch Pro-Jet I have has 3 mounting screws, so I tip the whole pickup away and get considerably crisper neck pickup sound with no flub at all.
HTH!