GEN734 … Foundation is the sign of the covenant of the bow [Both circumcision, (Genesis 17:10) and the rainbow (Gen. 9:12-13) are described as a covenant.  Furthermore, the word ‘bow’ is used in the Rabbinic literature as a synonym for penis] and the Bow of the Upper World is only outstretched in order to shoot arrows into the quality of Sovereignty, which is ‘the target for arrows,’ preserving the drop which shoots as an arrow to bring forth branches and to bear fruit. Now, just as the Supernal Bow is never stretched except it be toward this target, so, too, man should not stretch his bow and allow himself an erection under any circumstances except it be it for the proper purpose, namely, for his wife when she is clean, which is the time of union. And not more than this, for it causes a flaw in this quality, God forfend.  Exceedingly great care must be taken and the main precaution is to guard oneself from sexual imaginings.  CORDOVERO 112