Final answer:
The HDG REV switch on an aircraft's autopilot system is used to instruct the aircraft to turn 180 degrees and fly in the opposite direction.
Step-by-step explanation:
When you press the HDG REV switch, also known as the Heading Select Reversion switch, it typically affects the autopilot feature in an aircraft. Autopilot systems use the HDG REV switch to revert the heading direction to the opposite direction. For example, if you have a heading of 90 degrees, activating the HDG REV switch will change the heading to 270 degrees, effectively turning the aircraft around 180 degrees.
In most aircraft, this feature is used in conjunction with the autopilot for navigational purposes or to execute a standard rate turn. It's important to understand how to use this feature properly to ensure the safe operation of the aircraft.