The people of the village believed that twins were one soul split into two. Strangling one's twin was thought to 'unite' their souls and to make them become one. The energy unleashed during the ritual appeased the Hellish Abyss and prevented the Repentance from occurring.
When it comes time to perform the ritual, the twins are moved into 'Twin Houses', also called the Kiryu and Tachibana houses, where they will live in seclusion until the ritual. It is not stated how long prior to the ritual they are separated, however it is assumed to be a year, since if a ritual fails, it is held again a year later. Upon entering the houses the twins undergo a purification process to become a priestess in the case of Twin Shrine Maidens or priest in the case of Altar Twins. During this they become members of the Futago household. The twins then dress in white Kimonos, as in the traditional way of dressing the dead.
When it comes time for the ceremony, the twins are taken to the Kurosawa House via the underground tunnels by Veiled Priests.
The ritual process consists of the younger twin strangling the older twin while the priests beat staffs rhythmically. The younger twin is actually the first to be born, this is due to the fact that the villagers believe that the elder twin being stronger, graciously allows the younger and weaker twin to be born first. The older is sacrificed due to his/her greater strength. The mourners throw the body of the dead twin into the Hellish abyss to complete the ritual and appease the abyss.
The living twin will be left with a red mark on their throat, in the shape of a butterfly. Called 'The Remaining, their duty is to then watch over the village as a priest or priestess type role. While the dead twin would become a crimson butterfly, a spirit that acts as a guardian deity. In some cases, the remaining's hair may turn white, this is thought to be due to the shock of killing one's sibling. Afterwards, a Twin Deity Statue is erected in the village.
The ritual can fail in a variety of ways, the two noted in the game are; If the twins love each other too much, they may be unfit sacrifices for the Abyss, such as with the Tachibana twins. Or if the ritual is performed incorrectly, such as with Sae. Since she was killed alone, her soul was not reunited with Yae's. If this is the case, another ritual must be performed immediately, approximately a year later to avoid the Repentance. However, after two failed crimson sacrifice rituals in a row, the Repentance occurred and struck the village with darkness.
An alternate ritual, known as the 'Twin's Ceremony' is performed by other villages in the mountain and is a non-fatal form of the Crimson Sacrifice. The twins are dressed in white and tied by a red cord, connecting them as 'one'. They then conduct a séance and in the end become 'deities'. The ceremony is briefly mentioned in a Folklore book that Miku finds in Fatal Frame III: The Tormented.
While it is thought that many twins have been sacrificed over the years, (Due to the large amount of Crimson Butterflies in the village.) There are some known Sacrifices.