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Cuts the holder's Speed by 50% and negates all the holder's immunities to Ground-type moves, including Levitate and the Flying-type. Rotom-S is not an exception. The holder is also made susceptible to Arena Trap, Spikes, and Toxic Spikes. A Flying-type Pokémon holding an Iron Ball will receive neutral damage from Ground-type moves regardless of its secondary type.
Iron Ball is not exactly the best item to hold for most Pokemon—the Speed decrease and susceptibility to Ground-type attacks are almost never a good thing (unless you're running a Trick Room team, of course, but there are still better options to lower Speed, such as a Macho Brace). However, Iron Ball does have some utility if it can be Tricked onto the opponent. A notable user of this tactic is Metagross, which can lure in opponents such as Skarmory and Rotom-W and force the Iron Ball onto them, hopefully picking up Leftovers or some other useful item in the process. They are then susceptible to Metagross's Earthquake, with Rotom-W in particular taking super effective damage from it.
Either that, or if you are seriously ballsy you can give this item to Gengar in order to allow it to absorb Toxic Spikes. Some people do, apparently.
Battle Subway, 48 BP.
Pickup, Lv. 41-60 (1%).