Ice Screws
Like in all forms of roped climbing, anchors and protection are used to help mitigate risk in the case of a fall. When rock climbing, fancy cams and nuts can be used in cracks, while slings and be hooked around trees and rocks. When ice climbing, there is really only one place to anchor the rope to, and that is the ice. This is where ice screws come in.
Early ice screws (on the left), were more like ice nails, pounded in much like a piton. They were slowly developed into much safer and practical designs.
While it may be a bit hard to grasp at first, lead climbing is a very different concept than top roping at the gym, and is inherently much more dangerous due to icier nerves and longer falls. According to professional ice climber Will Gadd, about two out of every three falls leading on ice result in serious injury. This means that ice screws need to be able to withstand ample amounts of force in order to keep climbers safe.
Falling
In ice climbing, taking a fall while leading is one of the most scary and dangerous things that can happen. Will Gadd, one of the best professional ice climbers in the world, known for climbing Niagra falls and Helmcken falls, has stated "You should never fall while leading on ice." It has been estimated that two out of every three falls results in serious injury (Broken ankle, broken pelvis, broken leg, concussion, internal injuries), or death. DON'T FALL WHILE LEAD CLIMBING ON ICE.
In this diagram the belayer is attached to an anchor, while the climber is falling on a piece of placed protection. This information is vital because the anchor needs to withstand the force of both the belayer and the climber due to the tension forces in the rope. That is why it is pretty standard to use at least three screws in the belay anchor. Whatever protection the climber has placed highest, however, only has to withstand the fall of the climber, and that is why above the anchor typically only one ice screw at a time is placed.
Screw Angle
Ice screw placement is also vital to how much force the ice screw can hold. Conventional wisdom suggests that the screw be angled down, but in reality, that is not the case. One of the most studied aspects of climbing, MIT and Black Diamond have conducted intensive research on the force ice screws withstand.
The image above represents the different stress loads an ice screw applies on the ice at different angle in a 2003 study by MIT students. As you can see, with the lower angle, the ice screw is able to more evenly distribute the force. It is also important to note that rate of the force applied and quality of the ice are very important factors when determining if a screw will hold in the event of a fall.