You should know how to Half-Cab really well before trying this.
Setup like you would for a F/S Half-Cab except you should really wind your arms up, it makes it easier later. Pop down and immediately start spinning a Fakie 360. This is where your wound up arms come in handy. Unwind them really quickly and have them fully stretched out. As soon as then finishing spinning around tuck them in really close. This method is use by figure skaters to do things like quads (4 times around!!! That's a 1440!). Anyway so keep trying this until you get the spin about right and then work on the landing. You might need to spin a bit when you land on the ground but try to keep that to a minimum. When you land try to keep balanced and then slowly ride away.