Shevi'is Chapter 9, Mishnah 7. In the previous mishnah we established the dividing line of the second rainfall - when the second rainfall takes effect, when the second rain comes down - and we sorted through the views of the Tannaim and their reasoning as to exactly when during the month of Marcheshvan the middle rainfall occurs: on the seventh of the month, on the seventeenth, or on the twenty third, each according to the various opinions. Now that this measure of the second rainfall has been fixed, the mishnah presents a series of additional laws in which the second rainfall serves as the boundary line.
"Kayotzei bo" - the laws in which the second rainfall is the defining line:
One who rents a house to his fellow: He told him, "I am renting it to you until the rains" - what is the boundary line, and what counts as "the rains"? The mishnah says: "ad shetered revi'ah sheniyah" - until that second rain falls; that is what he meant.
One who imposes a vow on his fellow forbidding benefit: Someone who bound his fellow by a vow that he may not derive benefit from him, or who forbade himself from deriving benefit from his fellow, and stipulated that the vow would stand until the rains - here too the meaning is until the second rainfall falls, until that middle rain.
Poor people entering orchards: Until when in the autumn may the poor enter orchards to gather the gifts of the poor? Until the second rainfall falls. The reason: once the rainy season begins, they are trampling through a person's orchard and it becomes a problem for the owner. And once the rainy season reaches a certain point, it is already too late, since the ground is too wet, and the owner does not want people coming into his field and his orchard.
Straw and stubble of Shevi'is: From what point may one benefit from the straw and stubble of Shevi'is and burn them? From when the second rainfall falls, from when the second rain comes down.
An explanation of the law of straw and stubble of Shevi'is:
The straw and stubble of the Shemittah year are generally considered animal food, and animal food may not be burned or put to uses other than feeding animals, because of the sanctity of Shevi'is that is in it. But this holds true only as long as it is still fit for animals to eat; once it has been trampled many times and ruined by the rains and the like, it is no longer fit for animals to eat, and from that point on it may be used for burning and for other needs, such as making bricks and the like, as was done in Egypt. This is why the matter was fixed at the second rainfall: once the second rain comes down, the straw and stubble are no longer considered animal food, and they may therefore be used for other purposes.
In summary: After the previous mishnah fixed the time of the second rainfall, our mishnah teaches that this very same measure serves as the defining line in four laws: renting a house "until the rains," a vow forbidding benefit that was set "until the rains," the time during which the poor may enter orchards to gather, and the permission to use the straw and stubble of Shevi'is for burning and for other needs - since from that time on they are no longer fit for animal food.