In texts or speeches, the purpose of the author or speaker is always to explain a central idea that is related to a specific topic, situation or issue. Additionally, the central idea is developed through main ideas, which are more specific than the central idea and act as detailed points of it. For example, in a persuasive speech the main ideas are the key reasons that support the author's position, in the same way, in the case of a text that explains the effects of global warming the details or main ideas to explain the topic are the more important effects of global warming in terms of ecosystems, health, economy, etc. According to this, the main ideas there the ones that are detailed points of focus for developing your central idea.
Based on the information given, the rhetorical technique used is shift. It should be noted that a rhetorical shift occurs when the tone or the style of a speakers or a writer is being altered. A rhetorical shift is a can be signalled when transition words such as however, then, or but are used.
