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The holder's Attack 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 Band ignore Struggle and moves that are prevented from being used.
The move affected by Choice Band 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 Band 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 Band Jirachi is facing Choice Band 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 Band 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 Band is generally used for an immediate boost in Attack, and is the most powerful item to do it with. Choice Band is mostly used on more bulky Pokemon, as they will constantly switch out and take hits. The reason is that Choice Band locks the user into one attack, which an opponent will generally try to capitalize on. Fast and frail Pokemon can attempt to use Choice Band as well, as they usually have the Speed and Attack to do it reasonably well.
On a quick offensive team, Choice Band can provide some immediate power and force even more switches. It can also power ups moves enough to break through tough defensive cores such as Skarmory and Blissey. The boost in power will also give very few Pokemon opportunities to switch in, making it less of a big deal of being locked into one move. Choice Band also provides more power than two consecutive turns of an Expert Belt boost would, proving its effectiveness.
While it does provide a lot of power, Choice Band has a very bad side effect. Locking the user into one attack can be asking for a death wish, giving many sweepers a free turn to set up, especially if they are immune to the attack. Defensive Pokemon also enjoy the lock in effect of Choice Band, taking the time to set up entry hazards. Also, other moves such as Dragon Dance or Swords Dance will actually provide just as much or more power as Choice Band, but gives the user a freedom of moves. If Choice Band is Tricked onto another Choiced Pokemon, the user will still be locked into Trick.
Choice Band is usually best used on bulkier Pokemon who can take more hits. It should also be saved for much later in the game than other items, as it could be too easy to set up on early in the game. Choice Band does raise Attack 1.5x, which provides more than enough power to make up the drawback. As mentioned early, its damage output is greater than two consecutive turns of Expert Belt working.