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There is a vent on the gas tank that goes into a charcoal canister so that gasoline vapors do not vent to atmosphere, the CPC solonoid is hooked up between the charcoal canister and engine manifold. The solonoid is a Pulse Width Modulation (PWM, variable output) solonoid, turned on it blocks flow, turned OFF it allows. If the engine is warm, has been running for a set time, above a speed, and throttle is above a set point the ECM turns the solonoid OFF allowing the engine vacuum to suck the gasoline vapors out of the charcoal canister.