by 
The Curious Bag
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Most automatic-transmission vehicles have a stall (torque) converter of some sort. These fluid couplers allow an engine to idle without engaging the transmission, and are proven performers.
Purpose
A torque converter (TC) is used to keep an engine from engaging the transmission before it reaches a high enough RPM that it will move the car.
Parts
A TC contains an engine side fan (turbine) that pushes fluid through a transmission-side turbine.
Stall Speed
The TC is tuned so that at a certain RPM, the engine-side turbine overwhelms the transmission-side turbine's ability to pass fluid through without moving. The RPM at which this happens is called the stall speed.
Lock-Up
Most modern torque converters have a series of internal clutches that lock the engine to the transmission under cruise conditions, reducing fluid temperature and increasing fuel economy.
Failure
Torque converter failure usually manifests itself as "shudder," which feels like driving over a lightly wash-boarded road.
Source:
Rod and Custom Magazine: Understanding Torque Converters
Car Craft: What's up with Stall?
Chevy High Performance: TCI Transmission and Torque Converter Technology Insights
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