Here's what EMG say about the RPC:
The RPC is a variable active control that boosts the high frequency response of your guitar up to 6 dB at 4000 Hz, while simultaneously reducing low frequency response. Controlled via a pot, in place of your standard tone knob, this circuitry allows you to sweep from the natural sound of your guitar's pickup to accentuated highs typical of a single coil pickup.
Humbucker players who want a more Strat characteristic available will appreciate the RPC's ability to give new tonal options to your guitar. Like the SPC, it can be wired to one pickup or at the output of the guitar.