As hypnic jerks escape during the struggle between wake and sleep, the mind is undergoing its own transition. In the waking world we must make sense of external events. In dreams the mind tries to make sense of its own activity, resulting in dreams. Whilst a veil is drawn over most of the external world as we fall asleep, hypnic jerks are obviously close enough to home - being movements of our own bodies - to attract the attention of sleeping consciousness. Along with the hallucinated night-time world they get incorporated into our dreams.
So there is a pleasing symmetry between the two kinds of movements we make when asleep. Rapid eye movements are the traces of dreams that can be seen in the waking world. Hypnic jerks seem to be the traces of waking life that intrude on the dream world.Note to self: add Hypnic Jerks to the "If I ever started a band, what would it be called ..." list.