Zavim, Chapter Four, Mishnah Five
1) Ten cloaks one on top of the other, if he sat on the uppermost one, all are unclean.
2) A zav who was one one scale of a balance and in the other scale opposite there were objects fit to sit or lie upon:
a) If the zav pushed them up, they are clean.
b) But if they pushed him up, they are unclean.
c) Rabbi Shimon says: if there was one object it becomes unclean, but if there were many they remain clean, since none of them had borne the greater part [of the zav’s weight].
Section one: When a zav sits (other versions read sleeps) on a pile of something, all of the things upon which he sat become impure, even though he only touched the upper most one.
Section two: In mishnah 2:4 we learned that if another object causes a zav to be lifted up, that object becomes impure. So imagine a huge balance (maybe a seesaw?!) with a zav on one side and on the other side are things that one lies or sits upon. If he lifts them up they do not have midras impurity, because he didn’t “lean” on them. However, since he did carry them they do have some impurity, just a lesser degree.
If they push him up then he is considered as having leaned upon them, and they are impure.
Rabbi Shimon says that if there are multiple objects on the opposite side of the scale, then they have all divided up his weight equally. Since none of them bear the majority of his weight they all remain pure.