The parking brake is designed to hold the weight of the car on an incline. The engine is powerful enough to propel that same weight up the same hill and even gain speed while doing it. So the engine has enough power to overpower the holding force of the brakes and move the car. I should also point out that the parking brake is only engaging the rear brakes through mechanical force in most vehicles. When you step on the brake pedal you are applying all 4 brakes with hydraulic force that is also amplified by a booster.
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