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 moving extra quick, like falling from a ledge for instance.

Put another way; 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 so "painted" is also considered to be hit.
 * This is to compensate for the lag that comes with Internet gaming.
 * Works with the Battle Rifle, Sniper Rifle and Beam Rifle. This because they are ranged precision weapons.
 * Works regardless if you're scoped or not.
 * A.k.a Sweep Sniping and Drag Sniping (though only when it is used with Sniping weapons of course.)
 * Rainhailer adds: It is much easier to do higher sensitivities, because your reticle moves that much faster, though if you're used to playing with a lower sensitivity it will probably do you more harm than good.

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 just not be able to do it.  General advice is to go into custom games a lot of just snipers, you will get a lot better.  You can do it with stickies in a way that is a little weird I think.
 * 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 horizontaly 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. It is so most of the battle will be done close quarters. You will be moving more and trying to kill your opponent.

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. It really does not matter which direction you sweep, but I find left to right is the easiest. However, I have done it up to down and even diagonaly. Simply put, fire then sweep so the target goes red within a millisecond or two of firing.

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.