Yevamos, Chapter Five, Mishnah 6. This mishnah returns to an idea taught in the preceding mishnayos, but it draws a distinction - at least according to the first tanna - between a case where the chalitzah came first and a case where the act of marital relations came first.
The text of the Mishnah:
"Chalatz ve'asah maamar, nasan get u'ba'al" - the yavam performed chalitzah with his yevamah, and afterward made a maamar with her, or gave her a get, or lived with her.
"Ba'al ve'asah maamar, nasan get ve'chalatz" - the yavam first lived with her, and afterward made a maamar, or gave a get, or performed chalitzah.
"Ein acharei chalitzah kelum":
Once chalitzah has been performed, nothing that follows it takes effect. The chalitzah removes the zikah and the yevamah has left it, and therefore a maamar, a get, or marital relations performed afterward accomplish nothing at all. Even the maamar does not take effect, since we rule like Rabbi Akiva, who holds that kiddushin does not take effect with those forbidden by a negative commandment, and at this stage the maamar is nothing more than kiddushin. And he is permitted to marry the woman to whom he gave chalitzah.
The Mishnah emphasizes that this law applies "bein bat'chilah, bein ba'emtza, bein basof": whether the chalitzah was performed first, as the Mishnah states here; whether it was performed in the middle - such as where he made a maamar, then performed chalitzah, and then lived with her; or whether it was performed last. In any event whatsoever, once the chalitzah has been performed, any act that follows it does not count and has no validity at all.
Marital relations that came first - "there is nothing after them":
Marital relations, by contrast, "bizman shehi bat'chilah" - when they are the first act performed - "ein achareha kelum". The yavam has performed yibum with his yevamah and she is his wife in every respect, and any act performed afterward has no validity at all.
Marital relations in the middle or at the end - "there is something after them":
But if the marital relations took place in the middle or at the end - such as where he gave her a get, then lived with her, and then made a maamar, or where he gave her a get and made a maamar and only at the end lived with her - the other acts do in fact take effect. The reason: relations that did not precede the other acts are invalid relations, and the zikah of the yavam still remains. For that reason the maamar or the get are effective, and she requires chalitzah, since what took place here was not clean relations but relations that came after another act and were thereby rendered invalid.
The view of Rabbi Nechemyah:
Rabbi Nechemyah disagrees and holds: "Achas biah ve'achas chalitzah, bein bat'chilah, bein ba'emtza, bein basof - ein achareha kelum". In his opinion the concept of invalid relations does not exist at all in this context, and the very fact that another act preceded them does not invalidate them. The relations are considered a complete yibum, and that is what we look at: the yevamah becomes his wife, and any act that comes afterward has no substance.
In summary: In this mishnah we learned that according to the first tanna, chalitzah seals the process wherever it was performed - at the beginning, in the middle, or at the end - whereas marital relations seal it only when they come first; if they come in the middle or at the end, they are invalid relations, the zikah remains in place, the other acts take effect, and the yevamah requires chalitzah. Rabbi Nechemyah disagrees and equates the law of marital relations with the law of chalitzah: wherever they were performed, nothing after them takes effect.