Work hardening is the process of certain metals stiffening up as they are manipulated. Any time you are working with wire (bending, twisting, pulling, hammering, squishing) you are moving the metal's molecules and smashing them closer together. The closer the molecules are to each other, the harder the wire is to work with. This process intentionally changes the temper (hardness or softness of the wire) and forces it into a stronger state to not only ensure durability but also hold it's shape. The most commonly used wire tempers are dead soft, soft, half hard and hard. Hard wire doesn't need to be work hardened as it's already hard. You can work harden your wire creations before or after you're done manipulating. For example, it's easier to manipulate soft or half hard wire into the shape you desire but it may be too flimsy to stand up to extended wearing. You can then work harden the piece by using a mallet or hammer and bench block.