Final answer:
Pressing the EXPED pushbutton allows for an expedited descent when the selected altitude is lower than the current altitude.
Step-by-step explanation:
When the selected altitude is lower than the current altitude, pressing the EXPED pushbutton in the Flight Control Unit (FCU) initiates an expedited descent. This means that the aircraft will descend more rapidly than the normal descent rate. By doing so, the aircraft can quickly reach the selected altitude and maintain proper separation from other aircraft or avoid areas of turbulence or inclement weather.