Before us is Maseches Avodah Zarah, Chapter 4, Mishnah 5 (sections 4-6), in which we conclude the subject of the prohibition of deriving benefit from avodah zarah and of stripping it of its status - nullification. According to the Bartenura, the Mishnah contains two separate cases that are unrelated to one another.
The first case: an avodah zarah abandoned by its worshippers:
The Mishnah opens: "Avodah zarah shehinichuha ovdeha" - an avodah zarah that was abandoned by those who had been worshipping it until now. Here the Mishnah distinguishes between two sets of circumstances:
"Bish'as shalom - muteres": the abandonment was voluntary. The worshippers chose to leave it behind, moved on to their new home and left the avodah zarah there. This demonstrates that they are finished with it, that they no longer have any interest in it and no longer treat it with importance; therefore it has lost its status, and a Jew may use it and derive benefit from it.
In a time of war: the abandonment was forced upon them. The gentile fled for his life and left his avodah zarah behind, and that is no proof that he is finished with it and has nullified it. He may well come back and retrieve it later; he left it only because he had no choice, and that is not considered nullification.
From here we see that the act of nullification does not require any physical act performed on the avodah zarah itself. The very fact of relating to it as no longer being an avodah zarah, by abandoning it, is enough to constitute nullification and to permit benefit from it. This is strong proof that it is sufficient for the worshipper of the avodah zarah to say with his mouth, "I am finished with this thing," and with that the nullification is complete.
The second case: bimusiyos:
The second part of the Mishnah deals with an entirely separate matter - bimusiyos. We have already encountered this term itself in Chapter 3, Mishnah 7 (section 37). There the Mishnah listed three categories of objects: houses, stones and trees, with three cases in each. The last case of the stones (2c) dealt with a bimus - a stone that was originally an ordinary rock and was later converted to serve for avodah zarah.
The background to our Mishnah is the reality of those days: roads were few, and they served as the main arteries between the cities, unlike ordinary walking paths, since only a small number of chariots and carriages traveled on them. Along the roads people would set up pedestals, and when the king would pass by, the local priests would place their idols upon them, and the king would bow before them. This use was made only from time to time.
What, then, is the status of those pedestals? According to the Bartenura, the Mishnah's position is that they are not set up permanently to serve for avodah zarah; rather they are pedestals standing there for a variety of purposes and for whatever use may be needed, and only occasionally do they serve temporarily as platforms on which to place an avodah zarah. It emerges that this is the same type of ruling we saw in Mishnah 7 of Chapter 3: while the pedestal is serving as a stand for avodah zarah it is forbidden, but once the avodah zarah is removed from it, it goes back to being an ordinary pedestal for any purpose.
Accordingly the Mishnah teaches: "Bimusiyos shel melachim" - the pedestals set up for the needs of kings as they pass along the roads, in order to place an avodah zarah upon them - are permitted for a Jew to use at a time when no avodah zarah is resting on them. Their use is temporary and ad hoc only: if and when a king passes by, they will set the avodah zarah upon them, but the rest of the time they are not used for that, and that is not enough to give them the status of avodah zarah. It is nothing more than something temporarily subordinated to avodah zarah for the moment, like case 2c in Mishnah 7 of Chapter 3, and therefore a Jew may use them the rest of the year, as long as no avodah zarah is resting on them.
In summary: in this Mishnah we learned two separate cases. The first - an avodah zarah abandoned by its worshippers: in a time of peace this is a voluntary abandonment which constitutes nullification, and from here we see that nullification does not require any physical act on the avodah zarah itself; while in a time of war the abandonment is forced and does not constitute nullification. The second - bimusiyos of kings: pedestals that serve for avodah zarah only temporarily, and therefore are forbidden only at the time when the avodah zarah is resting upon them.