Jewish Mysticism
A person closes his eyes and places his hands over them, and appears as someone who has died, i.e., he is like someone without hands, eyes, or feet, and all of his actions are abandoned for the Lord… There is another hidden secret- at the moment one prays [the prayer of the eighteen benedictions] in his worship he intends the true unity, and the action that a person does below causes an act above, resulting in the copulation and union above. A person must hide out of shame for his Master and close his eyes in order not to look at the moment of copulation. Even though it is impossible to see above, “for no man shall see Me and live” (Exod. 33-20), nevertheless the knowledge of the sages constitutes their vision. On account of this they said that it is forbidden to look at the fingers of the priests when they spread out their hands, for the Presence rests on their hands at that moment, and it is forbidden to look at the Presence. Even though it is impossible to see, as we said, he must hide eyes.

Translated by Elliot Wolfson in Through a Speculum that Shines- Vision and Imagination in Medieval Jewish Mysticism, Princeton- Princeton University Press, 1994.