Final answer:
Connecting each section of a speech back to the central idea is a technique speakers use to develop their central ideas in speeches.
Step-by-step explanation:
A technique speakers use to develop their central ideas in speeches is to connect each section of a speech back to the central idea. This ensures that throughout the presentation, everything that is discussed is relevant to the main point being made and reinforces the theme for the audience. To do this effectively, speakers can create a purpose statement as a guide for constructing their speech. For instance, an informative speech may have a purpose statement like "By the end of this presentation, my audience will better understand the impact of plastic waste on the ocean." Every part of the speech, from the introduction to the conclusion, should then be connected to this central theme to maintain focus and provide clarity.