In reply to sutty: Sutty you have to think of ice as a laminate not as a solid, which is why ice dinner plates.
If you put the screw in downwards, when the weight goes onto it the outer layers of ice will most likely fracture, which means the screw is now 'sticking out a bit', this of course means the lever affect starts causing successive failures of ice deeper and deep until the screw pulls.
Puting the screw in angled slightly upwards, means the force from a fall is more like trying to pull the screw out of the ice straight rather than levering it through the ice. The threads on modern screws are so good that it is basically impossible to pull them out straight. The force doesn't fracture the ice in the same way. See www.bdel.com for the full results of their tests.