Zavim, Chapter 5, Mishnah 4. Our mishnah examines what happens when a tamei person and a tahor person come to rest one upon the other, and it teaches that even a partial contact of this kind is enough to convey tumah.
The rule of the mishnah
Let us follow the mishnah's wording clause by clause. "Miktzas tamei al hatahor": a portion of the person who is tamei comes to rest on the one who is tahor. "Umiktzas tahor al hatamei": or, in the opposite arrangement, a portion of the tahor person rests on the one who is tamei. The mishnah then extends the ruling to appendages. "Chiburei tamei al hatahor" covers the case where what rests upon the tahor person is one of the tamei person's attachments, and "vechiburei tahor al hatamei" covers the attachments of the tahor person resting upon the tamei one. For all four arrangements the mishnah issues a single verdict: "tamei".
Take a zav who rests upon a tahor person, or a tahor person who rests upon a zav, and the resting involves only part of the body. Perhaps it is no more than the zav's finger that comes to rest on the tahor man, or the reverse. Even if there was a stone or a beam separating between the two of them, so that they never made direct contact, the halachah is the same.
The mishnah then adds the chiburim, the parts of the body that are attached to a person without being flesh: hair, a nail, a tooth. If these attached parts of the tamei person rest upon the tahor person, or the attached parts of the tahor person rest upon the tamei one, the outcome does not change. In every one of these configurations, the one who had been tahor until that moment is now tamei.
Rabbi Shimon's distinction
Rabbi Shimon distinguishes according to which of the two is doing the resting. His opening clause reads "miktzas tamei al tahor, tamei". Should even a limb of the zav come to rest upon a man who is tahor, that man acquires tumah, and it is irrelevant that most of the zav's weight is being supported elsewhere. The principle is that a zav transmits tumah to anyone who carries him, and carrying a fraction of him already counts as carrying.
But the reverse case Rabbi Shimon treats differently: "miktzas tahor al tamei, tahor". If only part of the tahor person rests upon the zav, he stays pure. In this direction, in Rabbi Shimon's view, the tahor person contracts tumah only when the majority of his body is borne by the zav.
The halachah, however, does not follow Rabbi Shimon here. We rule as the anonymous first opinion of the mishnah: in either direction, and whether through the body itself or through the chiburim, and even with a stone or a beam interposed, the one who was tahor becomes tamei.