From Halopedia, the Halo wiki
Snap-to-target works similarly to auto-aim, but instead of slightly modifying the direction of the player's shots when they are fired, snap-to-target instead modifies the position the player is facing so that the weapon is facing towards the target enemy before the weapon is even fired. Whenever the player takes aim, either by using the analogue stick on a controller or through swiping their finger in a direction on a touch screen, if there is an enemy in that direction, within a small angle of error, the player will be snapped to face directly at that enemy, so that when they fire, the shots will be on target. The player will continue facing this enemy, even when moving, as long as the player holds the aim input in the direction of that enemy.
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