The flash bang is a different type of grenade that deals fatigue damage. In addition, they also reduce enemy weapon skills and have a chance to frenzy tunnelers. Upon detonation, the grenade releases a large explosion of blue colored energy.
Getting caught in a flash bang's radius yourself causes your screen to either blur or turn completely white (depending on how close you were to the blast). In addition your hearing is severely affected. In other words you are completely combat-ineffective until the effects wear off.
Can be found on marked hunters and marked scouts in varying amounts (1-3+).
Can be found on the occasional dead NCR trooper or NCR Ranger, typically in caves or other dark areas.
Can be bought when accessing the commissary terminal (Lonesome Road), which usually holds 5 each time.
Flash bangs were originally going to appear in the main game but were cut for unknown reasons. At first they were named stun grenades.
Flash bangs have a decent large blast radius, with the perk Splash Damage added, the radius becomes very large, and has a chance of inflicting the effects upon the player as well if he/she is not careful.