Negaim, Chapter 7, Mishnah 4. Our chapter opened with the words "Eilu baharos tehoros", and its first mishnayos listed baharos that can never become tamei: a baheres that was present on a person at a stage when he was not subject to the tumah of baharos at all, and which is therefore permanently removed from that parshah, together with the discussion of a baheres whose shade changed, where the halachah follows Rebbi Akiva that the nega is looked at afresh ("yeira'eh betchilah") and the person enters hesger. Our mishnah now moves to the ordinary course of a kohen's inspection: a plain baheres that carries no sign of tumah, and what happens when signs appear or disappear at the very moment the kohen is about to issue his ruling.
A baheres with nothing in it
The Mishnah begins: "Baheres ve'ein bah klum", a baheres that has nothing in it. That is, there is a white spot on the skin, but none of the simanei tumah are present: no black hairs that turned white (se'aros levanos), and no michyah, no patch of healthy flesh within the nega. In such a case the kohen does not declare anything final. "Betchilah, besof shavua rishon, yaskir": the first time the kohen comes and looks and finds nothing but the baheres itself, he shuts the man up for a week of hesger; and if he returns at the end of that first week and the picture is unchanged, he shuts him up for a second week.
"Besof shavua sheini ule'achar hapatur, yiftar": once the second seven-day period has run its course with no siman tumah having appeared, the kohen sends the man away tahor; and even afterward, should he present himself again with that same unchanged white spot and nothing new to show, he is again sent away tahor.
When the signs sprout while the kohen is speaking
The Mishnah goes on: "Odehu masgiro ufotro venoldu lo simanei tumah, yachlit". Picture the kohen mid-procedure: he is on the verge of declaring hesger, or on the verge of declaring the man clear, and while the ruling is still unspoken, simanei tumah emerge before him. It may be that a michyah shows itself inside the nega, or that a dark hair whitens; and when the inspection is the one that closes off the first seven days or the second, pisyon (a spread of the baheres) counts as the relevant sign as well. Because no psak had yet been issued, the kohen rules according to the situation now in front of him: "yachlit", he pronounces him a muchlat, a confirmed metzora.
The reverse case
The Mishnah now presents the mirror image: "Baheres uva'u simanei tumah, yachlit". Should a baheres come before the kohen at the very first viewing already bearing its simanei tumah, be it a white hair or a michyah, there is no hesger here at all. The kohen pronounces him muchlat immediately.
And what if the situation shifts while he is issuing the ruling? "Odehu machlito vehalchu lahen simanei tumah": the kohen is in the act of pronouncing him muchlat, and before the psak has actually been voiced, the simanei tumah disappear. Because no declaration was ever made, the man is in exactly the position of one whose baheres shows nothing at all, neither better nor worse, and so he follows the standard route: "betchilah uvesof shavua rishon, yaskir", he is shut up at the initial viewing and again when the first seven days close; "besof shavua sheini ule'achar hapatur, yiftor", and once the second seven-day period has passed with no hachlatah having been pronounced and no change in the nega, he is released as tahor, as he is also released after the patur.
An important contrast
The Rav draws attention to how different this is from a case where the hachlatah was actually pronounced. If the kohen already found the man to be a full metzora, sent outside the camp with the tumah of tzaraas upon him, and then the siman tumah disappears, the kohen declares him tahor immediately. Once hachlatah has taken effect, the vanishing of the sign is itself a sign of taharah. Here, by contrast, no psak was ever uttered, so the disappearance of the signs accomplishes nothing more than to leave him with a bare baheres, and he must go through the weeks of hesger like anyone else.