Zavim, Chapter 4, Mishnah 4. A zav has the power to turn the objects he lies or sits upon into an av hatumah, what we call tumas mishkav umoshav (midras). But this power has a condition attached to it: his weight has to actually rest on the object. More precisely, the greater part of his body weight must be borne by it. If only a portion of his weight presses down on an item, that item does not become tamei with midras. This principle is the heart of our mishnah, which explores case after case in which the zav's weight is divided up among several objects so that no single one of them carries most of him.
The first case: a zav lay stretched out over a row of five benches, "chamisha safsalim", or over five money pouches, "chamesh pundiyos". Was he positioned "le'orkan", running along their length? Then "tmeiyin", they contract the tumah. Was he positioned "lerochban", lying across their width? Then "tehorin", they are left pure.
The distinction is straightforward. When he lies lengthwise along the row, so that his body runs the length of each bench in turn, the weight of his body genuinely comes to rest on them as he shifts and settles himself, and each one carries him. When, however, he lies across their width, so that they are lined up side by side beneath him and his body crosses all of them at once, his weight is divided among them. No single bench ever carries the greater part of him, and therefore none of them contracts tumas midras.
This does not mean that nothing at all happens to those benches. They were touched by a zav, and contact with a zav renders an object a rishon letumah. That is a different and lesser status, governed by its own rules, and it should not be confused with becoming an av hatumah in its own right.
The mishnah then adds a case of doubt: "yashan, safek shenis'hapech aleihem, tmeiyin." If he fell asleep on them, they are tamei. Even where he lay across their width, once he was asleep we can no longer be confident that his weight stayed evenly divided. A sleeping person turns and shifts; perhaps he rolled this way, perhaps that way, and in the course of turning he may well have brought the bulk of his weight down on one of them. Since we have no way of resolving the doubt, all of them are treated as tamei, a tumah that rests on a safek.
A further scenario: "Haya mutal al shisha chiseiyos", he lay draped over six chairs, distributing himself so that each part of him rested on a different one. "Shtei yadav al shnayim", one hand on each of two chairs; "shtei raglav al shnayim", one foot on each of two more; "rosho al echad", his head on a fifth; "vegofo al echad", his trunk on the sixth. The ruling: "ein tamei ela zeh", not one of them is tamei except the chair "shetachat haguf", the one supporting his trunk. Hands, feet and head press down with only a share of him, split six ways, so nothing beneath them ever supports most of the man. The chair holding his torso does bear the bulk of his weight, and it alone turns into an av hatumah through midras.
The closing clause shifts from lying to standing: "Omed al shtei chiseiyos", he stood with a foot planted on each of two chairs. Rebbi Shimon's ruling here: "im rechokin zeh mizeh", if the chairs are set at a distance from one another, then "tehorin", neither one is rendered tamei by midras. Straddling the gap, he must hold himself balanced between the two, so his weight stays divided and no chair ever supports more than half of him.
The implication of Rebbi Shimon's qualification is that when the chairs stand close together, the ruling is reversed. A person standing with his feet close to each other naturally shifts from side to side, leaning his weight onto one foot and then the other. Since each chair is likely to have borne the bulk of him at some point, both become tamei with tumas midras.
The thread running through every clause of this mishnah is the same: midras tumah is not created by mere contact with a zav, but by an object actually carrying him. Spread that weight thin enough among enough objects, and no single object becomes an av hatumah; concentrate it, whether by lying lengthwise, by resting the torso on one chair, or by leaning onto one foot, and the tumah takes hold.