basically think of it in terms of a manual car, once you let your foot off the clutch it is locked to the engine. no slipping unless of course your clutch is junk. the torque converter is like that. it is designed so when your at idle in gear the engine can be "disengaged" from the transmission without going into neutral or the engine stalling. once you give it some throttle, the parts internal to the converter start to couple do to the fluid and propel you. then when you have reached an optimal point based on the pcm's programing, the converter which has an electric/viscous version of a clutch internal will lock, now the engine and trans are just like a manual. no slip at all. this helps fuel milage, and helps get the maximum engine power to the wheels.
much better explanation
http://en.wikipedia.org/wiki/Torque_converter
