The hand Tefillin are placed on the weaker hand, which is the left. There are seven bindings there, corresponding to the seven qualities. Each binding is regarded as ascents and descents, but at the same time they advance, meaning that in each wheel, as they go up, there must be a point of descent, or it is impossible to ascend. This is so because if there is no Kli [vessel], there is no light; if there is no lack, there is no filling of the lack.
Hence, the hand Tefillin are called Malchut, which is “faith,” in which there are ascents and descents. This is why it was said, “To you as a token and not for others as a token,” since there is still no revealing on the outside.
Conversely, the head Tefillin are regarded as the Torah, where there is revealing. This is why our sages said, “and they will fear you,” for it is already revealed outwards that the light is already clothed in the Kelim [vessels].