Cars with automatic transmissions have a device called a torque converter that connects the engine to the transmission.
Turning a car's engine crankshaft produces torque. Torque is what allows you to accelerate your car. The more torque an engine produces, the faster a car goes.
A torque converter is a type of fluid coupling, which allows the engine to spin somewhat independently of the transmission. If the engine is turning slowly, such as when the car is idling at a stoplight, the amount of torque passed through the torque converter is very small, so keeping the car still requires only a light pressure on the brake pedal.
When the torque converter starts malfunctioning, you may feel shuddering and even slipping in overdrive. You usually notice your car shuddering because it feels like it's vibrating. Your car will vibrate even when you're not going very fast. The shuddering makes the car lag and is very noticeable.
