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The user gains a 50% increase to Attack, but can only use the first move selected until it switches out or has its Choice Band 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.
Choice Band has had a huge impact on the game—many threats are now suddenly more dangerous because of increased attacking strength right off the bat, as opposed to taking a turn to set up. It can transform fast but weak Pokemon into serious threats, or slow powerhouses into battering rams. However, the drawback of being able to only choose one attack is a severe one—picking the wrong move allows an opponent to switch in easily. Although the drawbacks are difficult to work around, Choice Band still remains a fierce item for an offensive Pokemon, which all competitive battlers should be prepared for.
Choice Band's drawback can be quite severe, so most successful Choice Band users are those that can take the most advantage of the absolute immediate power, without the need to set up. With that in mind, the most feared Choice Band Pokemon are those that have powerful STAB moves, big Attack stats, and importantly, the ability to cause colossal damage with a single attack - to have an attack that few Pokemon will resist, or that will cause great damage even to those who do resist it. Haxorus is a prime example of this—with a base 120 power Outrage backed by a base 147 Attack stat, with the possible addition of a boost from Rivalry, it is capable of even tearing through Pokemon such as Heatran and Scizor, who resist the attack, with the addition of Choice Band. Some other noteworthy behemoths include Terrakion, who possesses both STAB Close Combat and Stone Edge in addition to a base 129 Attack stat and a base 108 Speed stat, and Heracross, who has STAB Close Combat and Megahorn in addition to a base 125 Attack stat. Note that while not essential, it helps if your Choice Band Pokemon resists Stealth Rock, as you will be switching in and out a lot, so minimizing passive damage can be helpful, another reason for Terrakion's effectiveness.
On the other hand, fast but weaker Pokemon can use Choice Band as a way of beefing up their offensive ability—while these Pokemon are less used typically, as Pokemon that are both fast and strong are not difficult to come by, there are some who require Choice Band in order to fulfil their roles on a team. Dugtrio is one of these; despite only possessing a measly base 80 Attack stat, its ability Arena Trap and access to STAB Earthquake enable it to perform a trapping role with considerable aplomb—to that end, Choice Band enables Dugtrio to obtain an OHKO on offensive Tyranitar as well as a 2HKO on some Blissey with Stealth Rock support. Other noteworthy Pokemon in the lower tiers include Scyther and Weavile.
Choice Band is not limited to these functions, either. One of the great Choice Band users is Tyranitar, who has great power behind its STAB Stone Edge and Crunch regardless, but also has a very useful tool in its STAB Pursuit. With the power of a Choice Band behind it, Tyranitar can essentially checkmate an opposing Psychic-type—regardless of whether or not it stays in, it gets KOed. It can also allow Tyranitar to break bulky Psychic- or Ghost-types that do choose to switch out, who would have survived otherwise. Of course, being locked into Pursuit is unfortunate, so beware the repercussions. Another notable Pursuit user is Scizor, who also has a few other tricks up its sleeve. The first is U-turn, which is harmless enough as a scouting tool when used by Landorus. However, when backed by STAB, Choice Band, and Scizor's base 130 Attack stat, it becomes capable of breaking opposing Pokemon on its own, while also allowing Scizor to escape danger. And of course, the infamous Bullet Punch—with Choice Band, STAB, and Technician behind it, Bullet Punch becomes capable of OHKOing frail Pokemon of its own accord, making Scizor an almost perfect revenge killer. Of course, being locked into Bullet Punch makes you perfect Magnezone bait, or vulnerable to being set up on, but hey, nothing's perfect. Speaking of tricks, one way to turn the negative side-effect of Choice Band to your advantage is to Trick it onto an opposing Pokemon that won't want it, for example a wall such as Blissey, or a special sweeper such as Starmie. Metagross is particularly good at this trick, as is Spiritomb; both of these Pokemon have access to Pursuit, as well.
In VGC and other fast-paced metagames, Choice Band is often less used than in singles, as there are fewer opportunities for switching out, and not being able to switch moves can be catastrophic. To that end, Gem items are typically favoured over Choice items, as despite the lack of a permanent boost to all moves, they do afford you the luxury of being able to switch attacks, which can indeed be invaluable.
Battle Subway, 48BP.