Iron Fist causes the power of affected punching moves to increase by 20% (approximated as a factor of 4915/4096 since Generation V). This boost stacks with the Punching Glove, resulting in a power boost of 32%.
Affected moves
The table below lists the moves affected by Iron Fist, as well as the Pokémon that can have Iron Fist and learn those moves.
Although the following moves are affected by the Ability, they currently cannot be learned by any Pokémon with Iron Fist. They may, however, be called via Metronome, Mirror Move, Copycat, or Mimic, or be used by a Pokémon that gained Iron Fist by an Ability like Trace or Receiver or a move such as Skill Swap or Role Play.
Headlong Rush might be considered a punching move due to its Japanese name, ぶちかましBuchikamashi (Headlong Rush), being a sumo term referring to the frontal collision of two sumo wrestlers right at the beginning of a match.