Answer:
Contrasts are one of the strongest techniques that good speakers use. They help you to keep the audience's attention throughout the performance. These can be chronological contrasts: the opposition of the past and present or present and future; emotional contrasts: juxtaposing jokes to serious facts or any other contrasts. In Obama's speech, you can find many examples of this technique. Thus, almost at the very beginning of his speech Barack presents two contrasts: "diversity of the United States" and "unity of the United States": It's the answer spoken by young and old, rich and poor, Democrat and Republican, black, white, Hispanic, Asian, Native American, gay, straight, disabled and not disabled. Americans who sent a message to the world that we have never been just a collection of individuals or a collection of red states and blue states. We are, and always will be, the United States of America.
