As we said last week, the gemara that says that a sick person may suckle from an animal if he needs fresh milk. The Ramban learns from this gemara, that a Jew may violate a prohibition using a shinui, an unusual way of doing it. Although milking an animal is Biblically prohibited, suckling directly from the animal is clearly an unusual way of milking. Based on this, the Shulchan Aruch rules that one may violate a Rabbinic prohibition using a shinui.
Although the gemara seems to allow even a Biblical prohibition when using a shinui, the Ramban clearly states that Rabbinic prohibitions were not waived, rather only prohibitions using a shinui. The Shulchan Aruch learns that this means that only Rabbinic prohibitions are waived and only when using a shinui. However, it is also possible to learn the Ramban that he that in order to protect the atmosphere of the day, it is only permissible to violate a prohibition if it is done in an unusual way, in order that it is clear that it is done for the sake of the sick person. Therefore, even a Rabbinic prohibition may only be violated with a shinui. The difference is that according to this understanding, even a Biblical prohibition may be violated with a shinui. The general rule of the Ramban is that there are two requirements: it must be a Rabbinic prohibition and it must be done with a shinui. However, even if is only Rabbinic because it is done with a shinui, it is still permitted.