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The user gains a 50% increase to Speed, but can only use the first move selected until it switches out or has its Choice Scarf removed. This boost does not count as a stat change. If this item is replaced with another Choice item, such as by Trick or Switcheroo, then the user will not be locked into their original choice. If the holder is locked into a non-attacking move and is Taunted, they will use Struggle. If the holder is runs out of PP for the attack they are locked into, they will use Struggle, but if the PP for that move is restored by an Elixir or Leppa Berry, they can select a new attack. If a Pokemon obtains a Choice item after using an attack, they will be locked into that attack. If Magic Room is used while a Choice-locked Pokemon is on the field, then the Pokemon can switch attacks, but once Magic Room ceases to take effect, they are again locked on the move they were originally locked in on before Magic Room was used.
In a metagame dominated by Speed and power, Choice Scarf stands out as one of the top items. Indeed, Choice Scarf is one of the reasons why Speed stats are less revered now than they once were—with an instant boost to Speed, Pokemon such as Tyranitar and Scizor are capable of outrunning even Pokemon such as Starmie and Tornadus, and if given to a faster Pokemon, such as Terrakion, they become almost impossible to catch. Hence, Choice Scarf has become the distinctive item of the revenge killer—the Pokemon equipped with Choice Scarf can come in, easily outrun the opponent, KO them before they can move, and switch out again—in fact, the negative side-effect of being locked into a single move hardly even matters, as revenge killers are designed to switch in, use one move, and switch out again.
There is more than one downside to a Choice item, however. The inability to switch moves is serious, so you need to know what you are doing when you switch it in, but it can be worked around. Arguably a more serious problem comes from the fact that the item provides no boost to the attacking stats at all, meaning that an offensive Pokemon's damage output is considerably lessened. Hence, the best Choice Scarf users tend to be those that can hit hard regardless—Terrakion is number 1 in OU, as not only does it have a base 129 Attack stat and access to STAB Close Combat and Stone Edge, it also has a base 108 Speed stat, which makes it nigh-on unbeatable in terms of Speed when equipped with a Choice Scarf—Jolly variants are even faster than neutral Venusaur with a Chlorophyll boost. Terrakion also demonstrates the second characteristic of a successful Choice user, that of being able to switch in easily, as it resists Stealth Rock and thus can switch in and out more times than something weak to it, such as Salamence.
Although fast Pokemon make great revenge killers with Choice Scarf equipped, this is not to say that slower Pokemon such as Scizor and Tyranitar do not do the job just as well, as with a Choice Scarf equipped they can outrun most of the unboosted metagame with ease. These slower Pokemon are often bulkier as well, and thus can switch in and out more easily while firing off powerful attacks. Tyranitar and Scizor are both, as previously mentioned, good examples of this, and indeed both of them also demonstrate other uses that can be made of Choice Scarf—Tyranitar can use it in tandem with its STAB Pursuit in order to checkmate a frail Ghost- or Psychic-type—since Tyranitar outruns them, they are KOed regardless of whether they switch out or not. Just be careful, as a Choice-locked Pursuit is a tempting prospect for a sweeper such as Terrakion or Lucario to set up on, and Alakazam is both faster than even Choice Scarf Tyranitar and carries Focus Blast. Scizor also has Pursuit, as well as STAB U-turn, which enables it to scout the opponent and escape before being touched.
Speaking of tricks, one way to turn the negative side-effect of Choice Scarf to your advantage is to Trick it onto an opposing Pokemon that won't want it, for example a wall or supporter, such as Blissey. Choice Scarf is one of the better items to use for this purpose, as it enables you to more often than not move before the opponent, meaning that the Pokemon will be locked into the attack that they used on that turn, so you will know for sure what they will use on the next turn. Notable Pokemon who make use of this strategy include Jirachi, Latios, and Starmie.
Although Choice items are not commonly seen in VGC and other fast-paced metagames, Choice Scarf is something of an occasional exception to the rule. While there are fewer opportunities for switching out, and not being able to switch moves can be catastrophic, the extra Speed provided by Choice Scarf can be vital in order to hit first, and can be made use of by nearly any Pokemon with a reasonable Speed stat. Of course, you have to balance the luxury of hitting first with the possibility of any subsequent hits being ineffective, so use with caution.
Battle Subway, 48 BP.
Castelia City - Talk to man in Pokemon Center after having acquired 30 different outsider ID numbers.