When a TC locks, it physically locks the input (engine) and output (tranny) together. Normally the input and output spin at different speeds, until the TC reaches "stall" speed. Now the newer TC's can make the engine and tranny spin at the same speed whenever it is needed.
I'm not too familiar with auto's, so I can't really tell you when or why it does it, but many people add a manual lock for dyno testing.
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