That is that He commanded us that a yevamah (levirate wife) release her yavam if he does not marry her. And that is His, may He be exalted, saying, "and she shall release his shoe" (Deuteronomy 25:9). And the regulations of this commandment have already been explained in Tractate Yevamot. And you already know their saying (Yevamot 39b), "The commandment of yibum (levirate marriage) takes precedence over release." And that is why [the tractate] is called Yevamot, even though it includes the laws of levirate marriage and of release, equally. (See Parashat Ki Tetzei; Mishneh Torah, Levirate Marriage and Release.)