תלמוד על בבא בתרא 3:3

Rav said, the essence of ḥalîṣah²⁷As the paragraph shows, this needs a semantic clarification since up to now, the word ḥalîṣah designated the entire ceremony by which the widow is freed to marry outside the family. But here, the word designates the action by which the sister-in-law slips the levir’s shoe off his foot, which must be followed by her spitting in front of the levir (Deut. 25:9). Rav insists that the widow’s action is invalid if it does not start with untying the shoelaces.
In the Babli, 102a, Rav insists that the deciding act is slipping the shoe off the levir’s heel. is the untying of the shoelace. Did not Rebbi Abba say in the name of Rav Jehudah, Rebbi Zeriqan turns to it in the name of Rav: The words of the Sages: If she slips off but did not spit, or spat but did not take off, nothing was done unless she slips off and spits²⁸Cf. Mishnah 3.. Even though Rav said “unless she slips off and spits” he agrees that the essence of slipping off is untying the shoelaces.