Using trekking poles as a brake, you're able to control the rate of decent. It's tricky to get the hang of the technique. The further down you go, the more speed you gain. As you attempt to counter-act an uncontrolled rate of a fall, you dig in with your poles (or axe). Go too far with your poles and you spin to get further out of control. If you don't dig in far enough, you continue to pick up speed. It's easy to get into a full blown cartwheel if you're not careful.