working principle of torque converter

I hope you can find your answer there

http://auto.howstuffworks.com/torque-converter.htm

Very simple description, you can think of a torque converter as a fan blowing against another fan (except with transmission fluid instead of air). The fan blowing is connected to the engine, the driven fan is connected to the transmission and ultimately the wheels. Because the driven fan is not directly linked to the engine, at lower rpms, the transmission can be stopped without the engine being stalled.