The Iron Nerve is the bloodline 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.
Memory Aid Edit
Since Iron Nerve allows for a specific type of kinesthetic, eidetic memory, it is possible to use it to replay IN user's side of an entire conversation.
This technique, however, would require both significant effort to learn, as well as assistance in form of using one's hands to help record it all. 
Social Iron Nerve Edit
Iron Nerve can replay any gesture or expression, as long as it was voluntarily made (puking uncontrollably won't be replayed). Thanks to this, Kurosawa are able to become terrifying diplomats and negotiators, showing exactly what they want to show.
This is however a clan secret that Hazou never got the chance to learn. At most, Hazou can improve his social acumen by training specific reactions.
The change in expression is not perfect- the muscles responsible snap into place, rather than smoothly transitioning. As such, it is possible to spot minute, unnatural changes, assuming good lightning and seeing it up close.