I can't understand a part of boeing's autothrottle system. Apparently the Asiana pilots couldn't as well, considering the 777 crash was caused by this misunderstanding. The problem lies in the 777's a/t HOLD mode, which is followed by IDLE mode. The 737 has the similar ARM, THR HLD, and RETARD modes. In these modes the servos are disconnected from the engines and the pilot can manually set thrust. Why? What is the purpose of disconnecting the servos? Thank you.