As we saw (section 1), the Torah prohibits doing melakha on Shabbat in a normal manner as the craftsmen did in the Mishkan, as it is written: “to work with every melekhet maĥshevet” (Shemot 35:33). However, if one performs the activity ki-le’aĥar yad (backhandedly), that is, with a shinui, he has not transgressed a Torah prohibition and he is therefore not subject to the punishment that the Torah prescribed for a Shabbat desecrator. Accordingly, it would seem that one may perform all the melakhot on Shabbat with a shinui. However, the Sages instituted a safeguard around the Torah and prohibited performing melakhot even with a shinui. For example, if one removes an object from a private domain to a public one in a normal manner – by carrying it in his hand or under his arm – he has transgressed a Torah commandment. However, if he carries it with a shinui – for example, with his leg, in his mouth, or on his elbow, ear, or hair – he has transgressed a rabbinic prohibition only (Shabbat 92a). Similarly, if one normally writes with his right hand only, and uses that hand to write on Shabbat, he has transgressed a Torah prohibition, whereas if he writes with his left hand, he has transgressed a rabbinic prohibition only (Shabbat 103a; below 18:2). One who cuts his nails with a pair of scissors transgresses a Torah prohibition, while if he trims them with his fingers or teeth he transgresses a rabbinic prohibition only (Shabbat 94b; below 14:2).
If two people acted in concert to perform one melakha that either one of them could have done on his own (such as holding a pen together and writing), neither person has transgressed a Torah prohibition, for it is written: “If any person from among the populace unknowingly incurs guilt by doing any of the things that by God’s commandments ought not to be done, and he realizes his guilt” (Vayikra 4:27). The Sages explain that “doing any of the things” refers only to doing the entire deed by oneself, not to doing only a part of it (Shabbat 92b). When two people perform a melakha jointly, each one is doing only part of the action. However, if they performed a melakha that neither one could have done on his own, such as moving heavy furniture from one domain to another, then they have each transgressed a Torah prohibition. If one of them is able to lift the furniture by himself but the other is unable to do so, the one who is able to carry it on his own has transgressed a Torah prohibition, while the one who helped but would not have been able to carry it on his own has transgressed a rabbinic prohibition only (MT 1:16).
The practical difference between a Torah prohibition and a rabbinic prohibition is that when in doubt about a Torah prohibition one must be stringent, while when in doubt about a rabbinic prohibition one may be lenient. Furthermore, in cases of need, when one is uncertain whether he may perform a certain action, he may perform it with a shinui. By doing so, the doubt now relates to a rabbinic prohibition, and therefore one may be lenient. (See sections 11-12 below, about the case of a shvut di-shvut [double rabbinic prohibition] when a mitzva is at stake or under pressing circumstances.)