Breaking the Impure World Reincarnation Contract is a special action available to one resurrected by the Impure World Reincarnation technique if they have retained their own personality and know the secret sequence of seals. It is not an independent combat ninjutsu, but rather a hidden capability within the resurrection technique itself.
After performing the required seals, the resurrected individual breaks the contract with the summoner. This grants them complete freedom of action while preserving all the properties of the resurrected body: invulnerability to ordinary physical damage, an endless supply of chakra, and the ability to regenerate. After this, the summoner can neither control such a resurrected individual nor cancel the technique to return their soul to the afterlife.
Madara Uchiha, who used this method, called such a capability "the only risk of this technique."