If you are in a room with an active shooter threatening you but the shooter is facing the other way, you have a chance to escape. The saying “I’m up, they see me, I’m down” is a phrase is used by the United States Army during Basic Combat Training instruction. The theory is that it takes an enemy at least four seconds to aim a weapon at a moving target, and six to aim accurately. By repeating "I'm up, he sees me, I'm down." while moving towards a known exit, the individual person keeps within the limits of the 'safe' exposure time frame.
As the person prepares to move from behind cover, they first selects their next position within a desirable distance from himself.
At "I'm up," the person gets up and begins to run.
At "he sees me," the person prepares to assume his next covered position.
At "I'm down." the person drops in place, situates himself behind adequate cover.