The "Effect" people are referring to is called Aim Assist in multiplayer. While you cannot "auto-target" the same way you do in the campaign, when you are looking down the sight in multiplayer with Aim Assist enabled, and an enemy runs across where the crosshairs are aimed the gun gets "dragged" in the direction that the enemy ran. This is supposed to be as if your character "noticed" the enemy and the mind acts faster than the hands so your character "subconsciously" begins aiming at it.
This feature can be good and bad. Good in the sense that it gives you a bit of a one-up on the enemy if you walk around aiming down the sight, but bad in the sense that if one enemy is standing still and you are shooting at him, and another enemy runs between you and the enemy you are shooting at your gun often gets "dragged" off of the enemy you were shooting at, but doesn't snap to the new target fully. This can result in 2 enemies getting notified of your presence and while you are trying to reacquire your original target he has already had the time to turn, fire two shots, and kill you.
Aim Assist can be enabled or disabled in the in-game options menu in multiplayer.
Pros do not use Aim Assist. Even if you don't play the game professionally, if you have gotten through to level 70 you should turn Aim Assist off. Anyone in prestige mode should not be using aim assist either.