The Auto Trigger makes the primary weapon auto-fire, which means that keeping the fire button pressed automatically makes the weapon fire until the player decides to stop. This doesn't affect fire rate. It also reduces damage dealt by 20%.
This attachment is useless on Machine Guns, Submachine Guns, and all automatic Rifles, Shotguns, Duals and Experimentals. It's highly recommended to equip the Gunslinger with this, as they use duals and sniper rifles, whhich usually have low RoF and are not automatic. Attaching this to non-automatic duals saves lot of time firing. Combined with the RoF Boost killstreak, it can make snipers very powerful. However, you shouldn't attach it to weapons that have low damage, because -20% side effect will make them very weak.