How do you optimally use Recon's "High Alert" part of the speciality?

I've been running Recon, but never even noticed the outside of the HUD flashing when an enemy is outside your view and is aiming at you. After doing some research on it, I'll now be paying more attention to the HUD flashing.

My question though is, how do you properly make use of it? Surely by the time an enemy is aiming at you from outside your POV, you're already gonna be dead. It doesn't seem like you'd have enough time to A) figure out exactly where the enemy is, and B) turning round and shooting them. Unless of course the HUD flashes in the exact direction to where the enemy is, but even then unless the player aiming at you is terrible, they're still gonna get the first shot off and kill you before you've got a chance to turn and shoot them back.