After the recovery of Unit-01 by Asuka, and the extraction of Shinji from the Core, the collar was fitted around his neck as a result of his actions which led to the Near Third Impact. Its primary function is to kill him if he loses control of his emotions again leading to the awakening of an Eva as it did during his fight with the Tenth Angel. This necklace is connected by a coded detonator that Captain Katsuragi holds.