Nedarim 75A

תניא כוותיה דרבי אמי? דקתני: בין שעשה בה מאמר ובין שלא עשה בה מאמר.

is it taught in the baraita in accordance with the opinion of R' Ami? The Talmud explains that support for R' Ami’s opinion is found in that which is taught in the baraita, as R' Akiva mentions the following distinction: Whether he performed levirate betrothal or whether he did not perform levirate betrothal. This indicates that R' Eliezer’s statement is referring to a yevama with whom he performed levirate betrothal.

אי נמי מרישא דקתני: משנכנסה לרשותו נגמרה לו. ואי דלא קדיש, מאי ״נגמרה לו״? תפשוט מיניה כשעשה בה מאמר.

Alternatively, support can be derived from the first clause of the baraita, which teaches: Once she enters his jurisdiction, she is fully under his authority. If the baraita is referring to a situation where he did not betroth her in levirate betrothal, in what sense is she fully under his authority? Deduce from it that the situation is one in which he has performed levirate betrothal.

מאי ״וכשאר דברים כן נדרים״ דקתני? אמר רבא, הכי קתני: אי אתה מודה שאין חייבין סקילה, כנערה המאורסה?

The Talmud then asks with regard to the baraita itself. What is the meaning of the phrase: And just as in other matters there is no such distinction, so too with regard to vows, that R' Akiva teaches in the baraita? Rava said: This is what R' Akiva is teaching: Do you not concede that one is not liable to be punished with stoning for adulterous sex with a yevama as he would be if she were a betrothed young woman? The status of the relationship is inferior to proper marriage, as one who has sex with a yevama does not incur the death penalty. Accordingly, the authority of the yavam with regard to vows is also inferior.