What it does
Yache Berry only covers Ice: an opponent lands a super-effective Ice move on the holder, it gets halved, and the berry is eaten.
Ice is the textbook offensive type — it punishes Dragon, Ground, Flying, and Grass. The classic picture: a Dragon eats an Ice Beam to the face and just dies; Yache halves it so the Dragon lives and blows the attacker away next turn. That's the signature use — letting a 4×-Ice-weak Dragon or Ground survive an Ice move and revenge-kill.
It's especially key in Dragon mirrors: a lot of Dragons are themselves 4× weak to Ice (Dragon/Flying, Dragon/Ground), and opponents keep an Ice move around specifically to remove your Dragon. Yache lets your Dragon eat that hit first and move first for the kill.
Same split from Focus Sash as always: Sash needs full HP and dies to chip and spread; Yache works from any HP but only covers Ice. If you know an Ice move is coming, Yache is the steadier pick.
Who runs it
The carriers are Dragons, Grounds, and Flyers for whom Ice is lethal.
Altaria (12%), Dragon/Flying and 4× weak to Ice, is the top user — one Ice move removes it, but with the berry it survives to set Tailwind or tank with Roost.
Flapple (8%) and Appletun (8%), Grass/Dragon, take Ice as a weakness and fear getting interrupted while stacking Dragon Dance or attacking. Noivern (4%) and Dragonite (1%), both Dragon/Flying, are 4× weak and rely on it to live.
On the Ground side, Gliscor (7%), Ground/Flying and 4× weak to Ice, is the textbook "only fears Ice" tank — Yache keeps it alive. Torterra (3%), Grass/Ground, and Hippowdon (2%), Ground, are Ice-weak too and take the berry to buy a turn.
How to use it
The rule: hold it for that one Ice move.
With your berry-holding Dragon or Ground out and an Ice attacker across from you lining up Ice Beam, Icicle Crash, or Freeze-Dry, eat the halved Ice, stay up, keep the turn. For your Dragon, surviving that hit usually means a Dragon STAB the next turn for the revenge KO.
In Dragon-vs-Dragon mirrors, Yache is the hidden decider: both Dragons fear getting removed by the other's Ice coverage, and whoever runs Yache eats one more hit and resolves the mirror first.
Note on Freeze-Dry: it's a special Ice move that hits Water super-effectively (a quirk), but against your Dragon/Ground/Flyer it's still just an Ice move, and Yache halves it the same way.
Tips & strategy
The big scenario is a 4×-Ice-weak tank like Altaria or Gliscor into an Ice move. The opponent clearly wants to KO you with Ice Beam and has the damage locked in — Yache's halving makes the 4× behave like 2×, which is often just enough to live. Your tank stays up and keeps working, and their Ice move is wasted.
Don't give it to anything Ice doesn't hit super-effectively. Yache only halves when Ice is super-effective; neutral or resisted, it does almost nothing.
Run the number first. Noivern, Dragon/Flying, is 4× weak but frail — a high-Sp.Atk Ice attacker at full power can overkill so hard that even halved it still KOs. Then don't bank on the berry; pivot out early instead.