Zavim, Chapter 4, Mishnah 3. The theme running through this chapter is heseit, the tumah a zav conveys by moving an object rather than by touching it. If a zav shifts something, whatever is lying on top of it is rendered tamei exactly as though he had taken hold of it himself. Our mishnah applies this to a zav who knocks or leans against an object with a loaf of terumah resting upon it, so that the loaf falls to the ground.
The determining factor is whether the object itself moved. Where the thing struck is firmly fixed and heavy, the zav's blow does no more than send a tremor through it; the loaf fell from the vibration, not from any movement the zav caused, and the terumah remains tahor. But where the object is loose, light or unsecured, the zav's knock genuinely displaces it, and the fall of the loaf is his own act. That is heseit, and the terumah becomes tamei. This mishnah lists the objects of that second sort.
The list of unsteady objects
"Hikish al hadeles": he knocked against a door, which of course swings.
"V'al hanager": the door bolt.
"V'al haman'ul": the lock, both of them movable fittings rather than fixed structures.
"V'al hamashot": the oar of a boat.
"V'al hakeles": the mill frame, which stood on the ground and through which the grain would pass, and which was readily moved about.
"V'al ilan shekocho ra": a tree whose strength is poor, a thin and feeble tree.
"V'al sucha shekocho ra", continuing, "al ilan yafeh": here it is the branch that is the weak element. The trunk itself may be perfectly healthy and solid, yet the particular bough upon which the terumah was resting has little strength of its own. When the zav bangs into it, that limb shifts, and down comes the loaf.
"Al sulam Mitzri", with the condition "bizman she'eino kavua b'masmer": the Egyptian ladder was the small, lightweight variety that a person carries from place to place, and it belongs on this list only in a case where nobody has secured it with a nail. Standing loose, it gives way as soon as the zav strikes it.
"Al hakevesh": a ramp or gangway, a platform of that sort.
"V'al hakora, v'al hadeles": a beam and a door which are simply resting where they lie, not set firm.
"Bizman she'ein asuyin b'tit, temei'in": the mishnah's qualification, and the key to the whole list. These beams and doors are counted among the unsteady objects specifically when they have not been fixed and cemented with clay. Because they are not held firm, the zav's blow moves them, and it is virtually as though he pushed the loaf over himself. Terumah lying upon them and falling is therefore tamei. What ruined it was not a tremor but the zav's own knock.
Chest, box and cupboard
"Al hashida, hateivah v'hamigdal, temei'in": if the zav knocked against a chest, a box or a cupboard, the terumah upon them is likewise tamei, for the same reasoning: these items are not anchored, so his blow amounts to heseit and the terumah is rendered impure.
"Rebbi Nechemya v'Rebbi Shimon metaharin b'eilu": Rebbi Nechemya and Rebbi Shimon disagree about these last three. In their view, if terumah rested on a chest, a box or a cupboard and fell when a zav knocked into it, the terumah stays tahor.
Understanding the machlokes
Rebbi Nechemya and Rebbi Shimon reason that these are bulky, heavy pieces of furniture. A zav who bumps against a chest or a cupboard does not actually budge it. Since nothing moved, there is no heseit, and no tumah has been transmitted. For them the requirement is precise: the zav must genuinely move the object, and here he has not.
The Tanna Kamma, the Chachamim, hold otherwise. In their view it is not decisive whether the piece is too heavy to shift. What matters is that a chest, a box or a cupboard is not fastened into the ground at all. Being relatively small and unattached, it is not in the category of a fixed installation, and the zav's blow against it counts as something close to a direct push rather than a mere tremor passing through an anchored structure. On that account the terumah resting upon it becomes tamei.
So the machlokes comes down to how heseit is defined: whether it demands actual movement of the object, as Rebbi Nechemya and Rebbi Shimon maintain, or whether it is enough that the object struck is unanchored and therefore the blow is treated as the zav's direct doing, as the Chachamim hold. And with that, the mishnah concludes.