What it does
Passho Berry only covers Water: an opponent lands a super-effective Water move on the holder, it gets halved, and the berry is eaten.
Water punishes Fire, Ground, and Rock. Water moves are already common, and against a rain team it gets worse — rain boosts Water power by 1.5×, so one Surf (the doubles spread Water move) or a boosted Hydro Pump on a Fire or Rock usually just KOs. Passho halves it, dragging that otherwise-lethal rain hit back into survivable range so you can still move.
That's the key use: a counter-pick insurance for Fires and Rocks that fear rain teams. In a game with no rain it's mediocre; against a rain team it can be the thing that keeps a key attacker from getting one-shot.
Same split from Focus Sash as always: Sash needs full HP and dies to chip and spread; Passho works from any HP but only covers Water. If you know a Water move is coming — especially under rain — Passho is the steadier pick.
Who runs it
The carriers are Fires, Rocks, and Grounds for whom Water is lethal.
Rampardos (11%) and Hippowdon (11%) are top users. Rampardos, a Rock glass cannon, hits like a truck but folds to anything — Water is a clean kill, so Passho buys it one swing first. Hippowdon, a Ground sand tank, takes Water as a major weakness and uses the berry to stick around for another sand turn.
Volcarona (9%), Fire/Bug, is 4× weak to Water (and to Rock), so a single Water move in rain deletes it — Passho is how it survives a rain team and stacks Quiver Dance. Rhyperior (7%), Rock/Ground, is 4× weak to Water for the same reason.
The Fires — Arcanine (5%), Incineroar (4%), Infernape (1%) — and Tyranitar (2%), Rock/Dark, all run it to survive rain.
How to use it
The rule: mostly hold it for rain teams, or any turn the opponent clearly lines up a Water move.
With your berry-holding Fire or Rock out, rain up, and a Water attacker across from you clicking Hydro Pump, Waterfall, or spread Surf, eat the halved Water, stay up, keep the turn.
Watch Surf in doubles: it hits both opponents, so if both your mons are Water-weak and standing together, Passho only saves the holder. Spread your Water weaknesses out, especially into rain.
The item trade-off: if you know a game has rain incoming and your Fire or Rock is a key attacker, Passho is worth more than an attacking item — it decides whether that mon lives through the first hit in rain. Swap back to an attacking item for non-rain games.
Tips & strategy
The big scenario is a 4×-weak Fire/Bug like Volcarona into rain. Rain's 1.5× plus a 4× weakness means even an ordinary Water move KOs it; Passho's halving makes the 4× behave like 2×, which is often just enough to live one hit and stack Quiver Dance to sweep back. That one berry can decide the game.
Don't give it to anything Water doesn't hit super-effectively. Passho only halves when Water is super-effective; neutral or resisted, it does almost nothing.
Run the rain number first. Something as paper-thin as Rampardos can still get overkilled by a boosted Water move into its weakness even after the halving — then don't bank on the berry to save it.