Torque converters are mated to automatic transmissions and serve two purposes: 1) to transfer the engine power to the transmission and; 2) to let the engine continue running when the car is at a stop. It uses a hydraulic fluid coupling to achieve this and is capable of torque multiplication when there is a difference between input and output speed.
There are three basic components to a torque converter: a pump, a turbine, and a stator. The converter housing is attached directly to the engine flywheel and the interior of the housing is lined with specially angled fins - these fins serve as the pump. When the housing spins, the blades fling the transmission fluid to the outside of the housing (due to centrifugal force) and thereby creates a vacuum in the center which is used to draw in more fluid. The fluid is pushed to the turbine, which is connected to the transmission. The turbine also has specially angled fins that are designed to change the direction of the fluid by the time it is exiting the turbine. As the turbine spins, it moves the input shaft of the transmission which then is transmitted to the wheels. The stator sits in the middle of the converter, sandwiched between the turbine and the pump. Its job is to reorient the fluid coming from the turbine (this fluid is now moving in the opposite direction of the pump) as it passes back to the pump.
A torque converter has three operating conditions: stalled, acceleration, and locked. A stalled condition is when the converter is spooling, but no transfer is being made to the transmission. Converters are rated at a certain stall speed, but this speed does not signify the point at which the car will move. Rather, the stall speed is simply the RPM at which the converter can no longer hold back the engine (can no longer stall the engine) if transmission output is prohibited (on the brakes).
The acceleration phase has the Challenger moving, but there will be a sizeable difference between pump and turbine speed - the pump will be spinning faster than the turbine (similar to a clutch slipping). Torque multiplication occurs during this stage.
Finally, the lockup or coupling stage is when the turbine is within 90% of the speed of the pump. A lockup clutch will actuate and lock the turbine to the pump, thereby creating a 1:1 spinning ratio. No torque multiplication is occurring as now the transmission is connected directly to the flywheel speed.