Here is a schematic from an Ibanez support page Eddie pointed me to
when I had questions about the Previous Owner of my JS
with the wiring.
The capaciter in question is the one marked 331.
As you roll the volume down, generally your treble gets choaked. The
cap bypass allows the treble to get through as the volume gets rolled
down, tricking your ears into thinking it's a more fuller sound.
It's an easy and cheap mod. A typical cap is anywhere from 3 cents to
25 cents. On the other hand, super duper high end gold plated in your
face build a $25,000 tube amp caps can cost from $5-$500. On a guitar,
with the cheap assed cables and noisy amps we love, I hardly think
you would notice the difference.
There is a LOT of magic that can be done by modding the electronics on
your guitar. But honestly, the easiest way to get a better sound, is to
practice, practice, practice.