Rice has two kinds of starches--amylose and amylopectin. Amylose is more prominent in long-grain rices and is the reason they cook up fluffy, distinct, and separate. Amylopectin, on the other hand, gelatinizes when cooked, and the short- and medium-grain rices used for risotto have an ideal quantity of amylopectin. Interestingly, the pricier carnaroli and vialone nano rices have relatively high amylose levels, and so will cook up more distinct and separate while still lending enough amylopectin to thicken the cooking liquid. We toast only half of the rice here, so that we can immediately "wash off" some starch from the not-toasted batch of rice when the broth is added. Constant stirring? Not really necessary. You'll see.