Chasidut zu Bamidbar 15:39

And therefore it is necessary to write tefillin on the hide of a clean beast specifically (SAOC 32:12), for the hide is the aspect of the imagination, which is the exterior of the intellect, the aspect of the intellect's clothing...For tefillin are such large “brains” that they have the power to clarify and repair the very Exterior of the Exterior, the Immaturity of Immaturity (Qatnuth) of the imagination, which is the main repair. For the greater the tzaddik, the more power that he has to clarify and rectify Qatnuth and Exteriority. And thereby all the worlds are repaired and all the Exteriorities and fallen souls have an ascent, by means of clarifying and purifying the utmost Qatnuth and Exteriority, which is where the main grip of the Qelipoth/Husks stems from. So now everything is clarified and nullified by this Tzaddiq, as mentioned. And this is the aspect of these tzaddiqim and elders, the least of whom remembers his umbilical cord being cut, which is the aspect of shrinking and cutting the umbilical, when one attains such perfect purity so as to remember what happened then. And this is the aspect of repair and purification of the skin, which is the connection between the mother and child, which is tied by the umbilical which is only skin. For, the devolution of the all the worlds from world to world is only from the “navel” downwards, as thoroughly explained in the Etz Chayim in Sha`ar Hanequdim and in many places, that the beginning of the lower world in the upper world is from the aspect of the navel downwards, where the aspect of NeHI [Netzach-Hod-Yesod] begins, the aspect of the upper world's “legs,” where the essence of the vitality of the world lower than it lies. And thus from world to world, from level to level. And therefore Hashem Yithbarakh created it such that the child is also physically tied to its mother's navel. For the devolution from generation to generation, from father and mother to son and daughter, begins from the navel downwards. And therefore the main drawing of the brains of these holy elders to us is by means of the least elder of them, who is the first elder from bottom to top, who related that he remembers the cutting of his umbilical as mentioned. For from there is the essence of the revelation of the brains and the devolution of the worlds from navel downwards as mentioned, which this is the aspect of drawing the light of tefillin, which extend down to the navel, which are the aspect of the straps that extend down to the navel as mentioned. But really, even though we only receive the light from the aspect of the navel downwards, by means of the last elder among them from top to bottom, who remembers the cutting of his umbilical as mentioned, all the vitality of the brains that we receive from there are drawn only from the highest elder, the first-ranking one of them, who is the aspect of this Blind One, who is this baby. For it is impossible to make vessels to receive the light below except by the power of the highest sanctity and the utmost supernal light.