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The user gains a 50% increase to Special Attack, but can only use the first move selected until it switches out or has its Choice Specs 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 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 on before Magic Room was used.
The least used Choice item, Choice Specs is still a powerful tool in the right hands. There are fewer hard-hitting special attackers, and even fewer candidates for an instant boost, but the few Pokemon that can make effective use of Choice Specs do so with fearsome power. 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, an equipped Choice Specs can make all the difference between an OHKO and a 2HKO.
As with Choice Band, typically the best Choice Specs users are those which can switch in and out easily, as well as having very little that can switch safely into their attacks (so large attacking stats, high-powered STAB moves and good single-move coverage are what we're looking for here). Hydreigon, Latios, and Heatran, to name a few, become horrific sweepers with the power to wreck teams when played correctly. Hydreigon in particular, with its base 125 Special Attack combined with STAB Draco Meteor, when boosted by Choice Specs, makes a powerful shoutback to SpecsMence of DPP. It is worth noting that any Choice Specs sweeper needs to be prepared to switch often—hence Hydreigon and Latios, who are immune to Spikes, make better Choice Specs users than Pokemon such as Volcarona, who has a 4x weakness to Stealth Rock and as such ideally wants to stay in for as long as possible.
Fast but weaker Pokemon can use Choice Specs 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 Specs in order to fulfill their roles on a team. Jolteon is one of these Pokemon, and with a Choice Specs equipped, the power of its STAB Thunderbolt is boosted tremendously, allowing it to act as a powerful revenge killer or cleaner. That's not its only trick, either—with access to STAB Volt Switch, Jolteon can threaten great damage to the opponent while at the same time escaping from danger, as well as enabling it to select a new move the next time it comes in, thus somewhat mitigating the drawback of Choice Specs. Other Electric-type Pokemon, such as Magnezone, Rotom-W, and Raikou are also capable of performing the same strategy.
Another notable trick to use is, of course, Trick—by switching your Choice Specs with the opponent's item, you can potentially cripple an opposing Blissey or physical attacker, thus turning the negative side-effect of the Choice item into something of an asset. Starmie is one of the best users of this strategy, though that's not to say that Choice Specs is not a good item for Starmie in its own right—with Choice Specs, Starmie's somewhat average base 100 Special Attack stat is made deadly, allowing it to utilize the full extent of its movepool, and even enabling it to 2HKO Blissey with Hydro Pump, if it has maximum Special Attack and Stealth Rock support. Alakazam is another noteworthy user of Trick, not least because it has Magic Guard, making it easily able to switch in and out without taking damage from Spikes or Stealth Rock.
In VGC and other fast-paced metagames, Choice Specs 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 favored 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.