by 
The Curious Bag
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Torque converters can best be understood by thinking of them as a flexible linkage between the engine and transmission. These couplers use fluid pressure to allow an engine to idle until it reaches sufficient rpm to engage the transmission. This rpm is known as the "stall" speed.
Term Origin
"Stall converters" are so-called because when the engine spins high enough, it engages the transmission, which will cause the engine to stall if it does not have enough power to overcome the brakes.
Torque Curve
Engines do not make the same amount of torque at every rpm the way an electric motor does. If a converter's stall speed is too low, the engine may not be making enough torque on engagement to prevent stalling.
Low Stall Speeds
Large engines destined for towing can benefit from a low stall speed. A lower-stall converter can help the drivetrain to better exploit a larger engine's low-RPM pulling power.
High Stall Speeds
Horsepower increases with rpm, so many performance enthusiasts prefer a higher-stall converter for better acceleration.
Stall-Speed Warnings
Higher-stall converters are usually smaller in diameter and produce more heat. Installation of an external tranny cooler is recommended where an aftermarket stall converter is used.
Source:
Rod and Custom magazine: Understanding Torque Converters
Car Craft magazine: What's up with Stall?
Chevy High Performance magazine: TCI transmission and Torque Converter Technology Insights.
Resource:
Torque Converter Video
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