Negaim, Chapter 8, Mishnah 5. The subject of this chapter is pericha, the case in which the baheres spreads out until it covers the metzora from head to foot. The governing principle is that everything depends on the ruling that preceded the spreading: when a full pericha comes after a kohen has already declared the man tamei, it is metaher him, and when it comes after a kohen has declared him tahor, it is metamei him. Our mishnah takes that principle and follows it through a whole series of reversals.
One point of background is needed. Rashei evarim, the extremities of the limbs (the tips of the fingers and toes, the ears, the nose and the like), have a special status. They cannot all be taken in by the kohen in a single view, and therefore a michya appearing on them does not render the metzora tamei. Nevertheless, they are fully capable of holding up a pericha: as long as the baheres has not reached them, the covering of the body is incomplete. The Tiferes Yisroel uses the term me'akev for this: they "hold back" the pericha. The two halachos do not contradict one another, since the leniency regarding michya belongs to a baheres ketana, a small nega, while pericha is a matter of a baheres gedola that must cover the entire body.
The opening clause of the mishnah is best taken in pieces. "Kol prichas rashei evarim": any instance of a baheres spreading out over the extremities of the limbs. "Shebifrichasan tiheru tamei": where that final stage of the spreading was what turned a man who stood tamei into a tahor. "Kshechazru, t'meiyim": should those extremities come back into view, meaning that the baheres retreats from them and they are once again bare skin, his tumah returns. The reason is straightforward. The pericha was effective only on the strength of there being nothing left uncovered anywhere on him. Once the tips of the limbs are visible again, the coverage is deficient, and the tahara that depended on it is gone.
The second half of the mishnah teaches the reverse movement: "Kol chaziras rashei evarim shebichazirasan tim'u tahor, niskasu, tahor; nisgalu, tamei; afilu me'ah pe'amim." Every case in which the reappearance of the rashei evarim rendered tamei a man who had been tahor (precisely the case just described in the first half of the mishnah), if the baheres afterwards covers them again, he returns to being tahor, since once more the pericha embraces his whole skin; and if they are uncovered yet again, he is once more tamei. And this holds even a hundred times over. The cycle simply repeats itself: covered and tahor, exposed and tamei, without limit.
The Rav explains that this is exactly the chiddush of the mishnah. It was necessary to teach us that the rule operates even many times over, as the mishnah itself spells out with the words "afilu me'ah pe'amim." One might have thought that after the status had already flipped once or twice, the man would settle into a fixed din; the mishnah tells us that there is no such thing. Each new state of the skin is judged on its own.
The Tiferes Yisroel understands the mishnah as doing something more: it is coming to define the case of the first mishnah of the perek. There we learned that when one of the rashei evarim reappears after the man had been purified through pericha, he becomes tamei. Which reappearance does that mishnah mean? According to the Tiferes Yisroel, only where those very rashei evarim had originally been the ones holding back the pericha.
He lays out the scenario step by step. A man was found with a small nega, and the kohen shut him up under hesger. When the kohen came to look at him at the close of the first week, or of the second, he saw that the nega had gone on to cover all of his skin, with only the extremities of the limbs still clear. Such spreading is a pisyon, a siman tumah, and so the kohen pronounces him a muchlat and he is tamei. The pericha was unable to purify him here, because the rashei evarim had not yet been reached and were therefore me'akev it. Afterwards the baheres pushes on and takes in those extremities too. At that moment the covering is whole, and since his standing at that moment was one of tumah, the completed pericha purifies him. This is the situation described by the words "shebifrichasan tiheru tamei": the pericha could only be metaher him because tumah was his prior status.
From that point onward the mishnah's two clauses describe the same man in his back-and-forth. When the extremities emerge from beneath the baheres, the pericha is undone and he is once again a muchlat; when the baheres closes over them, he is tahor; when they are exposed, tamei; and so on without end. For the Tiferes Yisroel the entire sequence rests on that opening stage, in which the rashei evarim, by remaining uncovered, prevented the spreading from taking effect in the first place.