At the climax of sexual intercourse the male tactically rests and nestles his phallus in between the buttocks of his partner. This is typically a follow up to the "doggy-style" position. Furthermore the ejaculate shoots at maximum velocity onto said partners back and/or hair.
This immaculate assembly received its name due to its strong visual resemblance to the great Gustav Cannon developed in the late 1930s by none other than the Krupp family.
ex. 1 -
'Jonathan assembled the Gustav with such pride, he watched in awe as fertilizer emitted from his cannon; showering his partner's posterior in warm viscous bliss.'
ex. 2 -
"Where's Sean....?"
"That slick bastard is up in his bunk assembling the Gustav again."