Sheviis, Chapter 2, Mishnah 2. This mishnah lists a number of activities that a person may perform up until Rosh Hashanah, with no requirement to refrain from them during the period added on to Sheviis. The reason: even if one were to perform these activities during Sheviis itself, they would be forbidden only by rabbinic law, and therefore the law of the eve of Sheviis does not apply to them.
"Mezablin u'me'adrin b'mikshaos u'v'midla'os":
"Mezablin" - one brings out manure and compost to fertilize the field.
"Me'adrin" - one digs with a ma'der, which is a type of hoe. The digging is done around the cucumbers and gourds, for their benefit.
These two activities are permitted in mikshaos, which are cucumber fields, and in midla'os, which are gourd fields, up until Rosh Hashanah.
"V'chein b'veis hashelachin":
This is true not only in a field containing cucumbers and gourds, but even in a beis hashelachin. A beis hashelachin is a field for which rainwater is insufficient, and which must be irrigated. Such a field may be hoed up until Rosh Hashanah, even if it contains no cucumbers or gourds.
Additional activities permitted during the period added on to Sheviis:
"Meyablin" - one removes the yabalos, which are a form of blemish on the tree.
"Mefarkin" - one removes the excess leaves from the tree, thereby easing the tree's burden somewhat.
"Me'abkin" - one places a certain type of dust on roots that have become exposed.
"Me'ashnin" - one smokes the tree, a process that was done in order to get rid of the worms in the tree.
All of these activities may be performed up until Rosh Hashanah.
The view of Rabbi Shimon:
Rabbi Shimon teaches that it is not only during the period added on to Sheviis that certain things are permitted, but "af notel hu es he'alin min ha'eshkol ba'sheviis" - a person may remove leaves from a cluster of grapes during Sheviis itself, and not only on the eve of Sheviis.
According to the first Tanna, removing the leaves from the tree - the "mefarkin" mentioned above - is permitted only on the eve of Sheviis. Rabbi Shimon comes and says that this activity may be done even during Sheviis itself, because it does not improve the tree but rather protects it from being ruined by an overabundance of leaves; and since it involves no enhancement but only the prevention of damage, it is permitted during Shemittah.
In summary: In this mishnah we listed the activities permitted up until Rosh Hashanah - fertilizing and hoeing in cucumber fields, in gourd fields and in a beis hashelachin, as well as meyablin, mefarkin, me'abkin and me'ashnin - all of which are forbidden during Sheviis itself only by rabbinic law, and therefore the law of the period added on to Sheviis does not apply to them. And we saw the view of Rabbi Shimon, that removing the leaves from the cluster is permitted even during Sheviis itself, since it involves no enhancement but only protection from damage.