Posted:Are you talking about a btb corkscrew in the sense that you are leaning waaaaaay backwards and spinning horizontally above your face and then under your back? If so then (since you are basically doing a windmill with a different body position) you only really need to know how to do a quad windmill...unfortunately I learned this move after getting the quad corkscrew (in front). So if you can do a normal quad corkscrew, bring it vertical to a quad windmill. Then do that whilst leaning back a lot.
Apologies if you are describing a different move from the one I'm envisioning.
~And when the day arrives I'll become the sky, and I'll become the sea, and the sea will come to kiss me for I am going home. Nothing can stop me now.~