Before us is Mishnah 11 in the fourth chapter of Maseches Eruvin, which deals with two opposite cases: someone who left the city's techum and found himself outside it, and someone upon whom nightfall came while he was outside the techum and who wishes to enter.
One who went outside the techum:
A person who is spending Shabbos in a city has a techum of two thousand amos from the edge of the city. If he went outside those two thousand amos - "afilu amah achas - lo yikanes" - even one amah, he may not re-enter. He cannot return into the city, and he has only four amos in the place where he now stands.
At first glance one might have argued otherwise, since part of the four amos permitted to him is absorbed within the techum itself. Even so, according to the first Tanna we do not apply the principle of 'havla'as techumin' here - we do not say that since the area in which he is permitted to walk is absorbed within the techum, it is as though he never left at all.
The view of Rabbi Eliezer:
Rabbi Eliezer disagrees and rules: "shtayim - yikanes, shalosh - lo yikanes" - if he went out two amos he may still return into the techum; if he went out three, he may no longer enter.
This approach fits with Rabbi Eliezer's consistent position, which we have already seen in earlier mishnayos: one who goes outside the techum and has only four amos stands in the middle of them, with two amos in each direction. And regarding absorption, Rabbi Eliezer holds the opposite of the first Tanna - 'havla'ah k'man d'malya': when the area in which he is permitted to walk is absorbed within the city's techum and overlaps it, it is as though he never left.
Therefore, one who went out a single amah still has one more amah remaining on the city side, which is absorbed into and overlaps the city's techum, and it is as though he never left. But one who went out three amos has only two amos in each direction, and they no longer reach the city's techum and there is no overlap between them, and therefore he may not enter.
One upon whom nightfall came outside the techum:
The Mishnah now turns to the opposite case: not one who is leaving the city, but one who is heading toward it. "Mi shehechshich chutz latechum" - a person upon whom Shabbos began while he was outside the techum - "afilu amah achas - lo yikanes" - even if he is only one amah away from the city's techum, he may not enter it, and he has only the four amos in which he is sitting.
This is the law even if he declared that his Shabbos residence should be within the city, and there are two reasons for this:
Establishing his residence in the city does not help him, since he is more than two thousand amos away from it.
Nor did he acquire residence in the place where he is standing, for had he established his residence where he stands, he would have had two thousand amos from that spot.
Since he has neither the one nor the other, he may not enter the city and is left with only the four amos of his own place.
Another view: fifteen amos:
Another opinion in the Mishnah rules: "afilu chamesh esreh amos - yikanes" - even one who is standing fifteen amos outside the techum may enter the techum and the city itself. The reason: those fifteen amos are in fact surplus.
Why? Because those who measure the techum do not extend the measurement all the way to its true end, "mipnei hato'im" - on account of those who err. They place the boundary marker fifteen amos before the point where the techum actually ends, out of concern for people who mistakenly wander outside the techum. It turns out that when one sees a marker outside the city indicating 'here the techum ends', there are in reality another fifteen amos of techum. Therefore, one standing within the fifteen amos adjacent to that marker is in fact standing inside the techum, and he may enter the city.
In summary: This Mishnah deals with two cases. The first - one who goes outside the techum: according to the first Tanna he may not re-enter even if he went out a single amah, and we do not apply 'havla'as techumin'; according to Rabbi Eliezer, at two amos he may enter and at three he may not, since in his view a person stands in the middle of his four amos and absorption is effective. The second - one upon whom nightfall came outside the techum: he has only four amos, because he acquired residence neither in the city nor in the place where he stands; and according to the other opinion, even at fifteen amos he may enter, since the measurers deliberately measure short on account of those who err.