A diz is used when you are combing the wool into a combed top. A combed top uses only the longest and strongest fibers and leaves the shorter fibers and noils in the comb. Combed top is best used for spinning worsted yarns.
Using the diz is the last step to making a combed top after the combing is complete. The wool is drawn off the comb through the diz, which is a disc or oval made of wood, plastic, or cardboard with a hole in the middle. Thread the fiber through the hole of the diz, and push the diz toward the comb as you draw the wool off the comb to form the top.