The heter mechirah – the sale of the land to a Gentile – was instituted by prominent rabbis as a temporary measure pertaining exclusively to rabbinical decrees related to the Shemitta year but not to Torah prohibitions. There are four specific types of labor that are prohibited by the Torah: planting, pruning, harvesting grain, and picking grapes. Some also say that plowing is a Torah prohibition. In practical terms, solutions have been found to allow pruning (by using a modified process) and also harvesting grain and picking grapes (within the framework of an otzar beit din – a "storehouse" belonging to a religious court). Two actions remain problematic: planting and plowing.
Before the Shemitta year of 5740 (1979-80), the Shemitta Committee of the religious kibbutz movement turned to the Chief Rabbinate asking how to perform agricultural work in fields that had been sold to a Gentile. Based on consultations with the Chief Rabbi at the time, Rabbi Shlomo Goren, and with professionals in the field of mechanics, the Zomet Institute developed the "Shemitton." This is an electrical gramma (indirect action) device which acts as a transmission for the pneumatic mechanism of a plow connected to a tractor. Pressing the "down" button causes a gramma mechanism to sink the teeth of the plow into the ground. This is permitted because plowing by the use of gramma is not prohibited by the Torah (and possibly not even by rabbinical decree), and it is thus permitted within the framework of the heter mechirah.
Plowing with a Shemitton has several limitations. In order that the plowing should be completely under the control of the gramma mechanism, the steering of the tractor must be automatic – in other words, the tractor must start moving along the track before the teeth of the plow are put into the ground (otherwise, starting the motion of the tractor would be considered as "active plowing"). As a precautionary feature, an automatic switch guarantees that the plow is picked up every time the tractor stops, so that it must be inserted into the ground by the gramma mechanism after the motion is again started. (A field that was plowed using a Shemitton usually has small "bald" patches, as a sign that Shemitta has been observed.)
When the Shemitta year of 5747 (1986-87) approached, the Shemitton was modified to include the additional labors of harvesting grain and picking grapes (these must also be performed without a person directly holding the pruning shears, which are operated by a gramma mechanism).
Because of the practical and halachic complications involved, it was eventually decided that the Shemitton should be used only in places where it is impossible to hire a Gentile to perform this type of labor.
More detailed halachic and technical explanations can be found in the following articles (in Hebrew):
Rabbi Yisrael Rozen and Yair Meir, "A Halachic-Technical Device for Shemitta
volume 2, page 428.
Rabbi Zeev Vitman, "Planting by Using a Gramma
Device during Shemitta
volume 7, page 53.