The fluid exits the turbine at the center, relocating in a different path than when it entered. The fluid exits the turbine going reverse the direction the pump (and motor) are turning. Slippage consumes energy throughout the coupling phase. Locking the converter shell to the turbine shaft helps prevent this https://torqueconverterclutch97418.blogoscience.com/40248068/the-basic-principles-of-torque-converter-clutch