Zavim, Chapter Three, Mishnah Three
1) If the zav and the clean person sat together in a large boat: (what is considered a large boat? Rabbi Judah said: one that does not sway with a man’s weight), or if they sat on a plank, bench, bed-frame, or beam when these were firmly secured;
a) Or if they both climbed a strong tree, a firm branch, or a Tyrian ladder, or an Egyptian ladder fixed by a nail;
b) Or if they sat on a bridge, rafter or door, when these were fastened with clay, even if only at one end, they remain clean.
2) If the clean one struck the unclean one, he still remains clean.
a) But if the unclean one struck the clean one, he is impure; for [in that case] if the clean one drew back, the unclean one would have fallen.
Section one: In all of these cases, since the things upon which the zav and clean person sat were stable, the zav has not moved the clean person. Therefore, the clean person remains clean. Note that most of this list is simply the opposite of those things listed in mishnah one. Either they were firmly secured, they were fixed by a nail or they were fastened in place by clay. In mishnah one, the opposite was always the casethe various items were loose. Therefore the pure person became defiled.
Section two: A clean person who strikes a zav remains clean because the zav has not moved the clean person. While it is possible that when hitting him the clean person leaned on the zav, since this is not certain, he remains clean.
However, if the zav strikes the clean person he is in a sense leaning on the clean person, for if the clean person moves when the zav strikes him, the zav would fall down (and be embarrassed). Leaning on the clean person defiles him.