During the Avoda¹ the Apter said² this is what I used to say. It is said that he actually verbalized the ineffable name of God. 

He related that he remembers the exact spot in the Holy Temple where he performed the service.

I.F.

R. Chaim Bick

1. I.e. the recounting of the sacrificial rites and purification rituals that were performed in the Temple by the High Priest on the Day of Atonement that is recited in the Musaf prayer on Yom Kippur. This section of the prayer is known as the “Avoda.”
2. In the section that recounts the confession of the High Priest: ְ וְכַךְ הָיָה אוֹמֵר. אָֽנָּא הַשֵּׁם “and this was what he said: I beg of You Hashem etc.” Instead of the referring to the High priest in the third person as is the standard practice, the Apter spoke in the first person, “this was what I said” etc. Thereby affirming that he was that in a previous incarnation he was the High Priest who performed the atonement rites in the temple.