What it does
Swarm boosts the power of the holder's Bug-type moves by 1.5× when its current HP falls to 1/3 of maximum or lower. The boost deactivates if HP recovers above the threshold, and reactivates if it drops again.
A concrete number: Heracross using Megahorn (base 120 power) at full HP already hits hard. With Swarm active at low HP, that effectively becomes a 180-power hit — matching what Choice Band would add, but conditional on being nearly KO'd. The burst potential is real.
Swarm only applies to Bug-type moves. Heracross also uses Close Combat — that gets nothing from Swarm. It's a type-specific boost, and Bug coverage isn't always what you're leading with.
Who runs it
Volcarona shows 0% Swarm usage in Champions, and the reason matters: Flame Body is its main ability. Swarm is Volcarona's hidden ability. The way Volcarona is built in Champions — setting up with Quiver Dance and then sweeping — doesn't fit a low-HP trigger. And Flame Body's 30% contact burn chance, while not a core mechanic, helps when physical attackers click Volcarona. So Volcarona picks Flame Body, not Swarm, essentially every time.
Scizor at 1% usage runs Technician as its primary choice. Technician boosts moves with base power 60 or lower by 1.5× — that covers Bullet Punch and Bug Bite with full uptime. Swarm requires being nearly KO'd first. The tradeoff isn't close on Scizor.
Heracross at 1% usage most commonly runs Moxie, which adds +1 Attack on every KO. A snowballing physical attacker values that more than a low-HP proc on Bug moves. Swarm appears occasionally on sets that specifically center Megahorn.
Ariados at 2% primarily uses Insomnia in Champions data. Swarm is not competitive on it.
Kleavor at 2% has Sheer Force and Steadfast available. Both outperform Swarm by a wide margin on a Pokémon whose best moves are Rock and cutting-type, not Bug.
How to use it
Swarm is mostly something you deal with situationally rather than design around.
If you're running a Swarm set intentionally, the play is to stay in at low HP and keep clicking Bug moves rather than pivoting out or using Protect. Opponents sometimes miscalculate damage assuming you don't have the extra 50% — if they expect a roll to miss the KO threshold and it doesn't, that's the value Swarm delivered.
In doubles, low HP also pairs with Flail or Reversal (both max power at 1 HP) on some sets. Those are Fighting-type moves so Swarm doesn't boost them directly, but they share the same 'low HP is an asset' mentality.
Tips & strategy
Always check which ability you're actually using. On every Pokémon in this list, there's a competing main ability that usually wins the comparison. Technician on Scizor is always on, not conditional. Flame Body on Volcarona provides passive contact deterrence. Moxie on Heracross stacks offensively without needing a crisis. Swarm comes up when the other ability genuinely doesn't apply — which is rare.
Low HP is a liability as well as a trigger. Once Swarm activates, priority moves, chip damage, and residual effects can finish you off before you act. You need to win the turn immediately, not set up for a future turn that may not happen.
Bug coverage is narrow. Bug hits Ghost, Dark, and Grass. In most competitive builds, those targets have other solutions that don't require you to be nearly dead first.