What it does
Hammer Arm is a Fighting-type physical move with solid base power. Every time you use it, your own Speed drops by one stage permanently (until you switch out or a speed reset clears it). Fighting covers Normal, Rock, Steel, Ice, and Dark — some of the most common defensive types in the game.
The Speed drop is the central decision point. In standard speed order, losing a stage makes you more likely to eat incoming attacks before you act. Under Trick Room, that flips: lower Speed means you go first. The same effect that's a penalty in normal play becomes an asset in room conditions.
Who runs it
Snorlax runs Hammer Arm at 26%, matching its Body Slam rate. At base Speed 30, Snorlax is near the bottom of the speed tier regardless — one stage less barely changes the math in regular play. Under Trick Room, though, it's the move that actively improves Snorlax's positioning. Fighting coverage against Rock and Steel types also matters: both resist Normal-type attacks, and Snorlax wants a way through them.
Rampardos at 14% pairs its enormous physical Attack with Fighting coverage to break through Steel-type walls. Speed 58 is already middling, so the cost is tolerable — but its low defenses mean that after one or two Hammer Arms, it's moving last and fairly easy to pick off.
Rhyperior at 5% barely registers a cost at Speed 40. Rock/Ground is its primary offensive profile, and both types get stopped by Steel. Hammer Arm is the breach tool.
How to use it
Two situations suit Hammer Arm best:
You're already slow and it doesn't matter. If the opponent is going to outspeed you regardless, the extra Speed drop is almost irrelevant. Take the power, deal the Fighting damage, and move on.
You're under Trick Room. This is the optimal home for Hammer Arm. Each use makes you slower, which under Trick Room means you keep going first. Snorlax in Trick Room can chain multiple Hammer Arms and actually grow stronger in the speed matchup with each use.
Don't use it when you've carefully tuned your Speed EVs to hit a specific speed tier — the drop will break that immediately.
Tips & strategy
Body Slam versus Hammer Arm on Snorlax is a teambuilding decision, not an in-battle one. If your team has Trick Room, Hammer Arm. If it doesn't, Body Slam is safer — same slot, no Speed liability.
One thing to manage: if Trick Room expires while you've stacked multiple Speed drops, you're suddenly one of the slowest things on the field in a regular speed order. Plan around when room ends and either keep it going or switch Snorlax out to reset the Speed drops before re-entering.