The "-C" flag stands for the long form "cpu-mining" command that would be in a JSON format configuration file.
That's what the arg is called but not what it does. I had assumed it did what it said but it just uses more CPU
to be more responsive to the GPU which helps the GPU hash faster. The CPU doesn't hash at all.
Just make sure you have one cpu core (including hyper-threading) per card or it can slow you down. I have an i3 with 6 750 ti cards and it slows down the hash.