In mythology Sin was born out of Satan's head. Sin and Satan had sex and gave birth to Death. Death then had sex with Sin (its mother) and gave birth to the hell dogs. From this metaphor Milton has made of raping the "hollow engines" with the "infernal flame" the product will be the hell dogs. I believe that the hell dogs are the "implements of mischief" that Milton refers to. I can envision these hell dogs made of fire jumping around biting and barking causing mischief especially to their enemies the angels.