The Jewish Time Traveler’s Manual

I have a bit of news your rabbi might have neglected to share with you: One of Judaism's major holidays is actually about time travel. No, really. Every Jewish holiday has a central ritual. Rosh Hashanah has the shofar. Sukkot has, well, the sukkah. Passover has matzo. After completing Tractate Yoma, the Talmudic volume about Yom Kippur, I am left wondering what exactly lies at the center of the Yom Kippur experience. Before you answer "fasting," undoubtedly the Yom Kippur ritual that we have learned to dread the most, understand that it is only given a few pages of mention within the entire tractate. Kol Nidre, the contemporary staple of the start of Yom Kippur services, is not even mentioned at all. What then lies at the center of Yom Kippur?