How many wicks does your hoop have? If your using 5+ wicks then you don't even touch the hoop to your back and you are actually just throwing the hoop across your back/chest, so it doesn't touch you like a normal chest/back roll. If you tried to roll your fire hoop like a normal chest/back roll you could damage your wicks or hurt yourself when it comes in contact with your body.
I use four wicks since it's way easier to learn the timing for breaks, throws, rolls etc. If you have more than five then it will be very hard to learn. Since I have four wicks it gives me more room between the wicks so it's easier to place on my back. it's really hard to explain what i do to make it happen so when I have more time I need to make a video for ya. It's all about practicing with your fire hoop as much as you do your normal hoop, learning how to slightly change normal tricks for your fire hooping, and being comfortable enough with it unlight to do the more daring tricks when it's light.