To do a half-cab you should have a 180 down pretty good. Think of your normal position on the board but roll backwards in it. As you roll backwards, bend your knees and wind up so you would be looking at your back foot. Pop the board and drag your foot up as you swing your arms and shoulders back into regulag position. You must then after you rotate a full 180, fully extend your legs to touch the ground and bend your knees with the weight of your body. You will roll away in your regular stance, or goofy stance if your goofy.