What it does
Psyshock is a Psychic-type special move in the strictest classification sense — it uses the user's Special Attack — but damage is calculated against the target's Defense, not Special Defense. That inversion is the entire reason to run it.
The Pokémon most hurt by this are the ones that deliberately stack Special Defense to check special attackers. A Assault Vest user with enormous Special Defense but middling Defense takes more from Psyshock than from a same-power neutral Psychic move. Blissey-type walls with extreme Special Defense/low Defense lose their defensive advantage entirely. In doubles, that wall-busting property matters: dedicated special tanks occasionally show up as part of Trick Room or support cores, and Psyshock goes straight through their armor.
Because it's a single-target move, Psyshock doesn't spread like Icy Wind. The pick is about targeting specific opponents, not general pressure. The decision is simple: if the target's Special Defense is substantially higher than Defense, choose Psyshock over a standard Psychic move. If Defense is higher, don't.
Who runs it
Espeon is one of the most consistent Psyshock users — high Special Attack means it actually threatens things with the move, and Magic Bounce makes it aggressive enough that opponents frequently try to out-special-wall it. Psyshock makes that plan fail.
Gardevoir uses it situationally, especially in mirror matchups where the opposing team has high-Special Defense targets that would shrug off a standard Psychic hit.
Reuniclus carries Psyshock in Trick Room builds — it moves first under reversed speed, and hitting Defense instead of Special Defense gives it reach against bulky support Pokémon that would otherwise sit comfortably behind their Special Defense investment.
Armarouge runs it to hit Defense-weak targets that might otherwise wall its special coverage, pairing with its Fire-type moves for two different defensive vectors.
Hatterene picks it occasionally to cut through Special Defense investment, though it more often leans on Fairy coverage.
How to use it
In doubles, the usage decision is mainly a team-building one. Identify the opponent Pokémon in your meta that run high Special Defense and relatively low Defense — those are Psyshock's targets. In practice, you use it exactly like any special move: click it into the right target. No additional decision-making at the battle level.
Under Trick Room, Psyshock on a slow, high-Special-Attack Pokémon like Reuniclus bypasses the Special Defense stacking that teams sometimes rely on to check special wallbreakers. Moving first and hitting Defense makes it genuinely threatening in a way a standard Psychic move sometimes isn't.
Tips & strategy
The choice between Psyshock and a same-power Psychic move comes down to one question: which of the target's defensive stats is lower? You can usually answer that in team preview and commit to the call before the match starts.
One mechanics note worth keeping straight: Psyshock is classified as a special move. It counts as special for Assault Vest blocking purposes, triggers Trace/Synchronize like a special move, and won't gain any physical-move boosts. It hits Defense — it is not a physical move.