When roasting coffee, the time between first crack and second crack, or first crack and the end of the roast (whichever comes first), is the critical phase that determines the flavor of the coffee. During this time, sugars and other compounds in the coffee bean cells are undergoing Maillard Reactions, which is essentially caramelizing. If you blast through this stage too quickly, the coffee will get burnt without ever developing sweetness. If you don't go far enough into this stage the coffee will be sour and underdeveloped, and lacking sweetness.
Whether you are looking for a light roast or a dark roast, it's absolutely critical that you do what you can to extend the Roast Development time.
The hard part, though, is that right at the first crack, the beginning of roast development, the beans go into an exothermic reaction. Where they had been absorbing heat from the roasting environment, now suddenly they are giving off heat to the environment like little exploding firecrackers. This raises the overall temperature of the environment, and tends to make whip you right through the Roast Development time faster than you wanted.
In order to compensate for this moment, anticipating the beginning of the first crack, it's a good idea for a roaster to back off the heat supply of the machine coasting into the first crack just right.
There's a lot more to it than that, of course. But that gives you an idea of the kind of thing we are talking about in my course this week.