Final answer:
The MAN ALT selector, when selected UP, typically causes the outflow valves of an aircraft to open, which reduces cabin pressure by allowing cabin air to escape.
Step-by-step explanation:
The question refers to the Manual Altitude (MAN ALT) selector found in aircraft, specifically the function of the outflow valves when the selector is set to the 'UP' position. When the MAN ALT selector is selected UP, it generally causes the outflow valves to open. This is done to reduce the cabin pressure by allowing more air to escape from the cabin to the outside, usually in preparation for descending or to correct an excessive cabin altitude situation.
It is important to note that specific procedures and functionalities can vary depending on the aircraft type and model; thus, pilots must refer to the Aircraft Flight Manual (AFM) or Operating Manual for their specific airplane to understand the exact behavior of the outflow valves in response to the MAN ALT selector.