If I'm not mistaken, the last thing in the world you want when going downhill is the rear wheels locking up. If the rear starts to slide, it will pivot on the front axle and you will end up backwards, if you can stop the vehicle from continuing out of control.
My cousins have a LOT more experience in snow/ice driving than I do, and they drive *much* quicker than I feel comfortable with in the same situations. Experience is hard to make up for.
Last year, I watched both of them go through a steep downhill covered in ice with no problems, while I and another vehicle spun out in the same place. I believe his spinout was caused by applying the brakes. However, in my case, I wasn't on the brakes, and was in 4low/low. About 2MPH.
I *have a feeling* that had I more tire/vehicle speed I probably wouldn't have spun around. My theory is that if the vehicle weight is great enough, and the surface slick enough, compression braking will STILL allow the tires to slide because the compression isn't letting them rotate fast enough, if that makes sense. Gravity is pulling the vehicle anyways, if that pull exceeds how fast the tires are wanting to turn, you will start to slide. My tires are AT's, both of their trucks have less agressive tires, which also probably helped, (more tire contact) but a spinout in my case *shouldn't* have happened, according to what you are told by most that know how to drive.
Typically, keeping vehicle speed down, off the brake, foot near the accelerator, and keeping as many tires as possible on "fresh" snow, or dirt, will keep you from having problems. Sometimes the best thing to do will be to hit the gas as you did, sometimes the best is to stand on the brake and hope stuff piles up against the tires, which is what stopped me from doing a complete 180. (after hitting the gas to keep from going off the opposite side of the road) Even with the ice, sliding sideways allowed the tires to gouge enough material to come to a stop.