if you mean throwing your back handspring on the beam, then listen to what these people have said.
but if you just mean throwing it on the floor--then there isn't that much else we can tell you.
if you know how to do one and your coach tells you you don't need spotted--then you're just going to have to GO FOR IT.
it's scary, but it's gymnastics. And once you do it, it'll feel great to get over your fear.
what's the worst that can happen?
i've seen a loooooot of girls fall on their heads doing back handsprings and getting up perfectly fine. They didn't even know they did anything wrong.
so don't work yourself up too much, okay? think of how great it will feel after you do it [: