What it does
Trace activates the instant the holder switches in: it copies the ability of the Pokémon directly across from it. In doubles, that means the Pokémon in the opposite slot. The copy lasts until the Trace holder switches out.
Some abilities can't be copied — Wonder Guard, Illusion, Trace itself, and a handful of form-change abilities like Stance Change all fail silently, leaving the holder with Trace unchanged. In practice this almost never comes up, but it matters against Aegislash.
Who runs it
Gardevoir runs Trace on about 27% of its sets. Its other options are Synchronize and Telepathy, both situational. Trace wins when the opponent's ability is worth copying — which against the right matchup it absolutely is. The Mega form (activated via Omni Ring) locks into Pixilate, so Trace only works on base Gardevoir. If you want the flexibility, you're staying base.
How to use it
The whole game with Trace is reading the opponent's team before the match and deciding which Pokémon you want to copy. High-value targets:
Intimidate — Gardevoir enters and also lowers the opponent's Attack. A free second Intimidate trigger your opponent didn't see coming. Swift Swim in rain — suddenly your base-80 Speed Gardevoir is running at 160, outpacing almost everything. Weather speed abilities are consistently the highest-value copies in a rain or sun matchup.
In doubles, the timing is critical. Lead Gardevoir against the Pokémon you want to copy, and the effect fires on entry — your opponent sees it happen but can't undo it that turn.
The passive risk: if your target switches out right after you copy, you're holding an ability that no longer matches the threat. That's Trace's core weakness — it's reactive, and a good opponent who knows you're running it will pivot away.
Tips & strategy
Before bringing Gardevoir into the lead, check every ability on the opposing team. If the strongest ability you could copy is Synchronize on their Umbreon, Trace is doing nothing for you this game and you're probably better off pivoting to a different strategy.
Trace resets every time Gardevoir switches out. Re-entering the field copies fresh from whatever is currently across from it — which can be a feature (copy a different ability on the second entry) or a problem (if you spent turns getting into position only to be forced out).
The 27% usage rate reflects that Trace is consistently high-value in specific matchups but dead weight in others. When it fires into something like Intimidate or a weather speed ability, it's one of the best single-entry effects in the game. When it doesn't, you're playing without a real ability all match.