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The holder's Speed is boosted by 50%, but it may not change moves until it switches out.
If the holder of this item uses Sleep Talk, it will select a random move for the first turn, and receive the boost as usual, but all subsequent uses of Sleep Talk or the move chosen by Sleep Talk will fail until the holder switches out.
In Diamond/Pearl, if the holder is brought in from a Pokemon that used U-turn to initiate the switch, and the holder has U-turn, it must use U-turn as its next move. If the holder doesn't have U-turn, it can freely select a move. However; in Platinum, if the holder is brought in from a Pokemon that used U-turn to initiate the switch they can freely select any move, even if the holder has U-turn.
Baton Pass counts as a switch as far as this item is concerned, and thus resets the move into which the holder is locked to no move.
The choice item effect doesn't ignore moves that can't be used because they have zero PP or no target.
The move affected by the choice items is not reset if the move runs out of PP and then has its PP restored.
Choice Scarf ignore Struggle and moves that are prevented from being used.
The move affected by Choice Scarf is not reset if the move runs out of PP and then has its PP restored.
The choice items share the same move; the effect is not removed if the holder loses one choice item and gains either the same item or a different one with Trick or Switcheroo.
The choice item effect also applies to the use of Pursuit as a Pokémon is about to switch. (In that case, the check for a choice item is made after Pursuit is used, not at the end of the holder's attack segment.) However, in the Japanese language version of Diamond and Pearl, if during the first use of Pursuit the opponent faints as it's about to switch, the holder is not prevented from choosing a different move next turn (assuming no moves were set by the choice items yet).
If a Pokemon with a Choice Scarf is Tricked by / uses Trick on a Pokemon that does not have a Choice item itself, the Pokemon that has the Choice item after the Trick will be free to use whatever move it wants on the next turn only if it went first. That Pokemon will be free to select whichever move it likes regardless of whether it was originally the Pokemon with the Choice item. The Pokemon that moved second will always be locked into whatever move it used on the turn it was Tricked if it is the one holding the Choice item after the Trick. However, if both Pokemon are holding a Choice item, and either Pokemon uses Trick, then both Pokemon will be locked in to whatever move they used on the turn of the Trick(s).
For example, if Choice Scarf Jirachi is facing Choice Scarf Bronzong, and Jirachi uses Trick while Bronzong uses Stealth Rock, Jirachi will be stuck using Trick the next turn, and Bronzong will be stuck using Stealth Rock and therefore have virtually no reason to stay in, meaning Jirachi, if it decides to stay in, will have no choice but to Trick the Choice Scarf onto the incoming Pokemon (unless it has Sticky Hold or Multitype). To expound on this, if the Bronzong user switches in Swampert to the Trick, Jirachi would be free to use whatever it wants. But if it uses Trick again and Swampert stays in to do whatever, then the next turn, Jirachi will be forced to use Trick, and not something like Reflect or U-turn.
Choice Scarf is generally used on Pokemon to boost their Speed and revenge kill much faster sweepers. It is preferred on stronger Pokemon that usually have counters that rely on outspeeding it. Choice Scarf does not boost Attack or Special Attack in any way, making it somewhat harder to effectively use.
Choice Scarf is a great item to use for revenge killing, the act of outspeeding and fainting a Pokemon after they have fainted one of yours. Choice Scarf also helps much slower Pokemon that lack Speed but have excellent Attack or Special Attack. Choice Scarf also gives Pokemon a chance to defeat fast set-up sweepers.
Unfortunately, using Choice Scarf, you will be locked in one attack with no boost in power. For this reason, attacks that have many immunities or resistances should be avoided, while powerful attacks that can hit many Pokemon reasonably well are ideal. However, many Pokemon will take the time to set up on you, especially when you can not hit as hard as possible. Moves such as Dragon Dance or Agility will actually provide as much or more Speed than Choice Scarf. Also, if you attempt to Trick Choice Scarf on another Choiced Pokemon, you will still be locked into Trick.
Choice Scarf should generally be used for late game where you can see which moves would be most effective in helping to take down an opponent's team. It should also be used on Pokemon that are either generally fast that could use more Speed to take down stat boosting Pokemon, as well as slow Pokemon who have high attacking stats.