Reflex Mode is a feature that appears in Metal Gear Solid V: Ground Zeroes and Metal Gear Solid V: The Phantom Pain. It allows the player a chance to stop a guard from causing an alert and works on all enemy soldiers.
Hideo Kojima's reason for including this is because the enemies in the two Metal Gear Solid V games can go anywhere in the area and are unpredictable. There is an option to turn this off and Kojima confirmed that the player will be encouraged to not use Reflex Mode and will get rewarded if they do not use it. It was first unveiled at the Ground Zeroes presentation for Tokyo Game Show by accident.