Here is a very short section of a starch molecule showing only five glucose links in it. Starch can be thousands or millions of glucose-links long. In solution, the long strands form helices. Chemists have long known that iodine likes to fit itself into those helices and make a very dark blue or black color. If amylase snips away at the long strands (see the arrows, below), helices cannot form and therefore no dark color can form. Thus the "iodine test" is a good one for determining whether or not there is amylase present.