Jump starting works by providing power to the car's battery, which then powers the rest of the electrical system, including the starter. If the starter motor itself is faulty (e.g., seized, burned out, or has internal problems), even with a fully charged battery, it won't be able to crank the engine. The jump start will provide power to the car's electrical system, but the starter won't engage to turn the engine over. You'll likely hear a click (from the solenoid engaging) or nothing at all.
