We are concluding the chapter (Chapter 12, Mishnah 6) on the topic of the tumah that applies to the participants in the burning of the burned bull and goat. The fundamental question here is: From what point is a person considered to be participating in the burning? This is not explicitly explained in the Mishnah itself, and therefore we will first present the two approaches.
The two approaches regarding what constitutes participation:
The view of the Tanna Kamma: From the moment a person leaves the Azarah carrying one of the carcasses, he becomes tamei. From then on, anyone who participates in the burning in any way - until the moment the carcass becomes completely ash and nothing remains of it - becomes tamei. The basis for this is in the exegesis of the verses.
The view of Rabbi Shimon: He disagrees and expounds the verses differently, maintaining that a person only becomes tamei during a very narrow window of actual participation in the burning. This window begins only from the time the fire has taken hold of the majority of the carcass, and it ends once the carcass has become black and charred and has lost its recognizable form as flesh, even if it is not completely ash.
The text of the Mishnah:
First, the view of the Tanna Kamma is brought regarding the people carrying the carcasses on poles: Imagine a sort of stretcher on which the carcass is placed, with two poles on its two sides carried by four people at the four ends of the poles. If the two front carriers have already exited the walls of the Azarah, and the two back ones are still inside the walls - the first two are tamei and their clothing becomes tamei, while those who have not yet exited the Azarah, even though they are carrying the carcass, do not become tamei until they too exit the walls of the Azarah. Once all four have exited, they all become tamei due to carrying the burned bull or goat.
All the commentators grapple with the question of what this sentence comes to add, since this was already stated in practice in the previous law. Perhaps the question is better than the answer. Among the suggested answers, the primary one is: If the carriers returned to the inside of the Azarah, or if it ultimately became clear that the animal was invalid and the like - this changes nothing, and their tumah remains in place.
The view of Rabbi Shimon:
Rabbi Shimon says: The carriers are not tamei at all. The ones who are tamei are only those assisting in the burning of the carcasses, and even they are only tamei from the time the fire has taken hold of the majority of the carcass - the participants in the burning at this point are certainly tamei. And from the time that "nittach habasar" - meaning that the flesh has become completely charred on the outside to the point that it no longer looks like flesh - "ein hasoref metamei begadav" - the one burning does not make his clothing tamei, and one who participates in its burning from this point onward no longer becomes tamei, since it is already burned.
The basis of his view is a minimalist approach to the verse "vehasoref osam yechabes begadav" - the one who burns them is the one required to wash his clothes, because he becomes tamei. "The one who burns" - specifically those burning and not the carriers; "them" - as long as they remain in their original form, as opposed to a state where they have lost their form.
In conclusion: The Sages disagree on this last point as well, saying: Even after the carcass is charred and turns black, as long as it has not completely turned to ash, one who participates in its burning from this point onward also becomes tamei. And the halachah is not in accordance with Rabbi Shimon, rather, the tumah applies during the entire period - from the moment the carcass is taken outside the walls until the last part becomes entirely ash.