Let me explain. One of the names of Yesod in Jewish Qabalistic texts is berith, the Covenant, and this represents the circumcision of the foreskin, as per Jewish tradition. The reason for its association to Yesod is due to Yesod's relation, on the anthropomorphized Tree of Life, to the phallus (this became later the "reproductive organs" to include the feminine, but traditional [some might say more sexist] views would have seen man alone here, and woman was seen, in her entirety, in Malkuth, as depicted in this post).
There are frequent references in the Hebrew Bible, however, to a circumcision of the heart. For example, in Deuteronomy 30:6 (NRSV, my italics):
"Moreover, the Lord your God will circumcise your heart and the heart of your descendants, so that you will love the Lord your God with all your heart and with all your soul, in order that you may live."
And in Leviticus 26:40-42 (NRSV, my italics):
"But if they confess their iniquity and the iniquity of their ancestors, in that they committed treachery against me and, moreover, that they continued hostile to me— so that I, in turn, continued hostile to them and brought them into the land of their enemies; if then their uncircumcised heart is humbled and they make amends for their iniquity, then will I remember my covenant with Jacob; I will remember also my covenant with Isaac and also my covenant with Abraham, and I will remember the land."
It is also echoed in later Christian works, such as those by Paul, where he states quite explicitly that "real circumcision is a matter of the heart—it is spiritual and not literal" (Romans 2:29, NRSV).
The heart is a reference to Tiphareth, and the circumcision of the heart is a reference to an act of Tiphareth, of the true covenant with God, the cutting away of the useless material that encases the heart, the abolishment, in a sense, of the ego. The circumcision of the foreskin in Yesod is but a reflection of this true circumcision, the circumcision of the heart, just as Yesod is the Moon and Tiphareth is the Sun, and the Moon shines only with a reflection of the light of the Sun. The circumcision of the heart is like the unfolding of the lotus flower, upon which Harpocrates, the child Horus (also attributable to Tiphareth, as both Child, King, and solar God) stands. Interestingly enough, however, this link with Harpocrates can be carried further.
One of the most well-known Signs used in the Golden Dawn tradition is the Sign of Harpocrates, or the Sign of Silence. While technically a misinterpretation of the ancient heiroglyph by the Greeks (it actually meant "child", not "silence", as was assumed from the "shushing" gesture), it has become a bit of established tradition, and is utilised to great effect within the Golden Dawn tradition. But how does this relate to circumcision or the Covenant? Because there is a third circumcision, a third Covenant, and this is berith leshon, the Covenant or circumcision of the tongue. This would be attributed to Da'ath, related to the throat and speech, and to circumcise the tongue is to, in effect, enact the virtue of silence. Thus it is also linked to Harpocrates, via the Sign of Silence. Indeed, even the circumcision of the flesh could be seen as linked to him, as he is a child, and it is as a child that the circumcision of the foreskin occurs in Jewish tradition.
Thus, Harpocrates is the secret to all three circumcisions, all of which are attributed to Sephiroth on the Middle Pillar, barring the two outer-most Sephiroth of Malkuth and Kether, which are, in a sense, reflections of each other. Ultimately, however, the circumcision of the heart is the focal point for most magicians, for it is there that the magical child sitting silently in the hidden lotus within all magicians finds its first blossom and finally unfolds. This is, in a sense, the magical puberty that will lead to the magical maturity of adepthood.