Face painting, also known as sweep sniping, drag sniping, and sweeping, is the act of firing while quickly moving one's aiming reticule over an opponent's head, in a linear motion. If a player sweeps their reticule in such a manner while firing, and the target's head happens to pass under the reticule, then the shot will count as a headshot.
The trick works especially well when you or your target is moving very quickly, like when falling from a ledge.
When a player pulls the trigger in Halo 2 or later games, the game remembers where the reticule was just before and just after the shot was fired. The game then connects these two points with a line. Anything that happens to be on this line ends up being hit by the shot. This is used to compensate for latency and lag; without such compensation, it would be nearly impossible to accurately fire upon an enemy in an online multiplayer match.
Moving the reticule in a linear motion while firing increases the distance between these two points, allowing the shot to cover a larger area. Furthermore, by moving the reticule such that a target (like an opponent's head) rests on the "detection line", players can hit a target without needing as much precision. The trick works well with any ranged precision weapon, including the two Battle Rifles, the Covenant Carbine, the Sniper Rifle, the Beam Rifle, and all Magnum models.
Higher sensitivities can further increase the distance between the two aiming locations, effectively lengthening the "detection line"; however, players who are used to lower sensitivities may not wish to increase their look sensitivity.
In shieldless gametypes, or when facing an enemy with low shielding, another technique is to "circle-sweep". Simply move the reticule reasonably close to the target's head, and quickly move the aiming thumbstick in a quick, tight circle while firing. This tends to work best with extremely high look sensitivities (such as 10).