This paradoxical passage locates "the firmament of heaven" in the "final" or most earthy part of the human body, the genitals. The end is the beginning; the lowest is most high.
Our physical world is the lowest of all dimensions. Yet, because the desires implanted within physical human beings are the most intense, the physical world has the power to reveal the greatest amount of spiritual Light. When we transform our most extreme desires from negative to positive, we create Light for all the worlds. Our most powerful instinctive desires are our sexual impulses. When these desires are tamed and directed towards spiritual endeavors - sharing love with your spouse, bringing down positive energy from above, bringing forth new life - the Light aroused is of untold luminance. In this way, the wisdom and strength to utilize our sexual desires for positive purposes is instilled within us.