this was discussed before, kinnda vague, its a clipping thing, it cuts frequencies, and lowers output i think, to get an overdriven sound.
someone did a bunch of tests with it, and posted sound clips, let me search
the suggested rating on the pot they give is probably just for the black ice itself.
it is just a pair of diodes that limit the output. when the pickup puts out enough voltage for the diode to "turn on" it allows a path to ground. this puts a cap on the max amplitude of the signal, when the pickup hits this amplitude the peaks are essentially cut off.
a lower value volume pot limits the pickup output. mostly at the resonant peak where voltages are likely to be higher, this may limit the ability of the black ice to overdrive.
if you chose to use one, get those pickups really close for more output and use a 500k on the volume, 250 on the black ice. if you have a tone you may like a bigger cap with it.