Quiz: Remember the definition of mal de mer?
A scheme to maximise use of a communications line by giving a larger share of the bandwidth to the modem at the end which is transmitting the most information.
Only one end of the connection has full bandwidth, the other has only a fraction of the bandwidth. Normally, which end gets the full bandwidth is chosen dynamically.
Asymmetrical modulation was made famous by the HST mode of the early high-speed modems from US Robotics.