The invocation option 'In a drop down' lets you expand and collapse the embedded chunk in the topic.

Invocation option 'In a drop down'

In this example we have created a few 'sections-of-interest' in a bulleted list by means of embedded chunks that the reader can open and read. When the topic is shown in the browser, the embedded chunks are by default hidden.

Embedded chunks in a bulleted list

When you click the hotspot, the embedded chunk is shown. When you click the hotspot again, the embedded chunk becomes hidden once more. A drop down typically pushes the paragraphs after the hotspot downwards in the topic as indicated in the next image.

Embedded chunk shown in a drop down