Bold and in quotes: Original text of Baal HaSulam
Regular: Commentaries of Rav Laitman
Capitalized italics: transliteration from Hebrew
It is said: ""The Lord is thy shade." If one thinks, the Creator too thinks of him."
A person doubts the Creator and hence, in accordance to this sensation, it seems to him as though the Creator doubts him as well.
"...and when the Creator thinks, it is called "the mountain of the Lord." (in Hebrew, doubts are called Hirurim; mountain is called Har) This is the meaning of, "Who shall ascend into the mountain of the Lord, and who shall stand in His holy place?"
Who will ascend into the mountain of doubts that the Creator erected before him? There is only way to ascend into this mountain: in spite of the obstacle, cling to the Creator more. And when the next obstacle comes one's way, cling to the Creator even more, for there is no other way to escape obstacles.
The Creator created our desire, the Creator made filling for this desire, and we are absolutely unable to determine anything either in these desires or how they get filled. We can only perform indirect actions: define our relation towards the Creator and thus save ourselves, as does a little one in relation to a big one. Thus with the help of our doubts, we approach the adhesion with the Creator.
Thus, it is said: "Who shall ascend into the mountain of the Lord, (the question is who?) and who shall stand in His holy place? (Who will reach the entire height of the spiritual degree of the Creator?) He that hath clean hands."
Naturally, it does not happen in the beginning, because at first we are full of filth - desire to receive for our own sake. Later, however, we acquire the correction called with "clean hands". Hands are our will to receive; they become clean after we cleanse them with the help of restriction, when we no longer wish to use our will to receive for our own sake.
Hands are clean through correction ""But Moses' hands were heavy,..", by holding up our hands towards the Creator.
If we hold up our hands we prevail; if we put them down we lose the war. Hands are our faith. And faith is Bina, the power of bestowal. And if the hands possess the power of Bina, the power of bestowal, all the vessels of reception are incorporated into Bina, bestowal, and hence, we succeed.
Correction makes hands clean ""But Moses' hands were heavy..." "Clean hands" is when hands are cleansed with the light of Hassadim, the property of bestowal.
"...and a pure heart," is the result our work in the heart (Avodat Liba).
This means that in addition to performing a restriction on his vessels of reception and building GE, incorporating them into Bina, now he acquired a courageous heart (Bar Levav), meaning he prevailed over his heart.