Shaving the head and growing the nails of the Yefas Toar – its spiritual counterpart:[1]
In relation to how the female captive of war is to be treated if desired in marriage by her captor, the verse instructs that her hair is to be shaved and her nails are to be grown out. In spiritual terms, this refers to the following: The hair of the head represents the unhealthy overflow of intellect that has fallen into material matters, while the nails represent the unhealthy overflow of kindness that has fallen into the side of evil. [Meaning, that when a person uses his intellect and emotions of love for engagement in material matters more than which is necessary, such as if he drowns his mind in work and business, and drowns his emotion of love into fulfilling his material lusts and passions, then this is considered that he has overflowed his holy and spiritual aspect of intellect and kindness into the external side.[2]] One is hence instructed to remove this overflow to prevent it from reaching the sides of evil. You see, while in the side of holiness which is above Seder Hishtalshlus [i.e. above Atzilus], a surplus of holiness remains an actual aspect of holiness, and does not feed the side of evil, nonetheless, in the side of holiness of Seder Hishtalshlus [i.e. in Atzilus] the surplus can serve as a source of nurture for the unclean side.
[1] Likkutei Torah p. 36b
[2] See Biur below in F!