You can learn this trick once you're confident doing the Corkscrew (see Lesson #5).
The Vortex involves moving the hoop from Lasso down to waist height (but not touching the waist), transferring it to the other hand behind your back and lifting it back up to Lasso again in a continual motion.
Again, I will explain this trick for those who hula hoop to their left (anti-clockwise), so if you hula hoop to the right reverse the hand and turning instructions.
Start with the hoop in Lasso in your left hand. Turn on the spot to your left and bring the hoop down over your head to waist height (similarly to the Corkscrew), but do not let go of the hoop or let the hoop touch your waist. Instead, transfer the hoop to your right hand behind your back and, while continuing to turn to your left and leading with the elbow, lift the hoop back up to Lasso.
The Vortex takes a lot of practice (and a lot of hitting yourself in the face)...but once perfected it looks very graceful and impressive.