The Iron Nerve is the kekkei genkai of the Kurosawa clan, distantly related to Uchiha clan's Sharingan. It affects the user's motor nerves, allowing them to perfectly replicate any past action, and subconsciously enhances their physical skills. Offspring have an additional 1/32 chance of receiving the Sharingan.
The Kurosawa have genealogy records going back 10 generations (about 200 years). All of those generations had the Iron Nerve, and no one is sure exactly when it sprang up. 
Iron Nerve users can perfectly replicate any past actions; this allows them to draw perfect seal blanks every time if they've done it once, greatly reducing the risk of infusing an incorrect blank. It also gives them a mental library of seals, and allows them to copy other seals by merely observing them for a few moments.