I was curious about this too and PM'd Pixiecurl to find out.
She described her method:
Using the bowl of the diffuser to scrunch up a section to the root and then turning on the heat, once that section is dry diffusing with cool air to set it, then switching of the hairdryer before moving the bowl away.
Basically it means you arent blowing your curls all around with random air flow.
It seems to work for lots of curlies on her and so got aptly named the pixiecurl method.
I think she said she does it upsidedown too but I'm not sure.
My description probably isn't that great sorry.