Face painting

Face Painting is when you quickly pass your reticule over an opponent's head, creating a line across an imaginary plane. If the opponent's head happens to be in that plane, it counts as a headshot. These work especially well when you or your target is moving extra quick, like falling from a ledge.

Summary
Alternately stated: At the very moment you pull the trigger, Halo 2 remembers where the reticule has been just before that and also where it goes right after. Thus, the reticule is "painting" for a very brief moment. Everything that was "painted" is also considered to be hit. It is much easier to do higher sensitivities, because your reticule moves that much faster, though if you're used to playing with a lower sensitivity, it will probably do you more harm than good.
 * This is to compensate for the lag that comes with Internet gaming.
 * Works with the BR55 Battle Rifle, BR55HB SR Battle Rifle, Covenant Carbine, M6C Magnum, M6D, M6G, Sniper Rifle and Beam Rifle. This is because they are ranged precision weapons.
 * Works regardless as to whether you are zoomed/scoped or not.
 * Also called Sweep Sniping and Drag Sniping (though only when it is used with Sniping weapons).

Tips

 * Either you do it or you do not. You have to be good at it as well. Once you are a good sniper you could start doing it because once you try to change your style early you will be unable to do it. General advice is to go into custom games a lot with only snipers. Hopefully you will get better. This also is possible to perform with Plasma grenades but this seems to be an anomaly.
 * Falling from heights you tend to move faster than the normal walking speed, so the imaginary "line" that you create is more dramatic. If you are physically moving faster, then you will cover more area during this span of time, and your shooting plane is much wider. The Battle Rifle is another good weapon to face paint with. Try using the combo and use the face paint technique to get your headshot with the Battle Rifle.
 * Just sweep across your head with a three round burst. You shoot horizontally at the persons head. It is easy to learn with the Battle Rifle because of the three round burst. A good way to practice with the Sniper Rifle would be to lockout all snipers. If so most of the battle will be conducted as close quarters combat.

Walkthrough
It is quite simple. All you have to do is fire then sweep your crosshair over an enemy (preferably the head, hence the name face painting). It has to be done in a very short time however, almost simultaneously. Basically fire and sweep and as long as the crosshair turns red within a few milliseconds of firing, you will get the hit, and if it is a headshot, then you get a kill.

Different Methods
In shieldless gametypes, or against an enemy with low shielding, another technique is to "circle-sweep" (recommended Look Sensitivity is ten). All you have to do is get your reticule somewhere reasonably close to a head, rotate your looking stick in a quick, tight circle and you will have an easy headshot.

Note: This method might not be practical on lower sensitivities.