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Killing 20 Overlords in a Minute Slows Production.
Or.. if you prefer.. Using scouts A La Zileas.
Scouts are one of the most mobile units in the game, and in terms of air to air power, are the fastest way to move a huge punch anywhere. However, most protoss dont realize the power of scouts, or for that matter max out the power of scouts. Scouts are for quick strikes on key targets, as well as support for heavier units vs land. They also are great for killing stragglers of any variety, and killing transports in a single salvo, and the Protoss' first line of defense against guardians.. Scouts are cheap to upgrade.. make sure you do so. Also, they are nearly worthless without the speed upgrade and to a lesser extent the sight upgrade.
Basics of Scout Control
Scouts are rarely effective against land targets (or even air targets) in groups less than 5. However, in larger groups they indeed own a lot of things. The biggest things to remember when controlling scouts are:
1) Don't take on heavy anti air defenses with them. Have a reaver do it. If theres long range anti ground units (tanks, templars) feel free to rush in and kill the unit then run out quickly and put the scouts out of range on hold position.
2) Use shift queing against certain units. In particular swarms of other air fighter units, and weak anti air land units. Shift queing is essentially you do a normal attack on one unit (not an area) then hold shift while right clicking on the units you want to kill after this. This gets an OPTIMAL fire distrubtion, and you dont sit around beating up 10 units at once, you concenrtate firepower on one unit at a time and slowly decimate them that way. This is much better because you reduce the firepower of the opposing forces much faster, thus taking less damage. Units that should be shift queed against are ghosts, wraiths, small groups of battlecruisers, other scouts, carriers, hydras, marines in small numbers, goliaths, and mutalisks. This technique is also fun to use against other things, but units that are a direct threat against the scout are most important.
3) Dont make scouts win air to air fights. Let them clean up. This means, always have shuttles with templars handy. A full group of scouts is powerful, but scouts are VERY expensive and its foolish to take on 30 mutas with 14 scouts. Sure youll win, but youll lose most of youir scouts. If you mix in a few templars and start psi storming, youll take probabaly no losses, and basically do most of the damage with psi storm, then finish off the mutas with scouts. Often its ok to NOT shift que when you do this because you want all units being attacked kinda. Even battlecruisers take mondo damage from psi storm. Keep this in mind.
4) Dont send scouts into battle without full shields. Retreat scouts that are taking insane ammounts of damage to a shield battery, and maintain groups of shield batteries at tactically useful locations. this means, dont be afraid to set up an outpost (not to be confused with an expansion) nearby the point of attack consisting of some shield batteries and a cannon or two. If you can keep these nearby and in a defended expansion, even better yet.
5) Hit and run a lot. Once your presence is felt, it wont be long till an army shows up. You dont want to be around when the army shows up. When you hit, do a surgical strike and kill a bunch of producers, or ONE key structure, then run like mad. My personal favorite is running into a zerg base, ignoring the spore colonies and ripping up a spire in like 10 seconds then running like mad.
6) Kill spellcasters even if it involves taking some hits. If you lose some shields so what! A full mana spellcaster is worth a lot more than their base cost. Templars and defilers especially are high priority, as are science vessels. Ghosts and queens are important to a lesser extent.
7) You can widdle down large air fleets with hit and run attacks, particuarly if you out range them. A spread out pack of mutas will lose a muta per salvo of missles, so you may as well hit and run this way.. this can work as well TO A CERTAIN EXTENT with the slow bcs if you hit the edge, but you aught to be very careful about that.
So. In conclusion:
Dont make scouts take damage when another unit can do the job better.
Hit and run a lot. Kill targets of opportunity with abundancy.
Scouts should take advantage of shield batteries more than any other unit.
Carriers suck because they lack fast attacking and retreating :P
Templars win fights. Scouts are the clean up crew.
Shift queing is your friend.