During a recent support call, it occurred to me that I had spent so long working on Harmony, and was so familiar with it's premise, that I had failed to properly communicate it fundamentals. So, included in the Harmony 2.4 help manual is this description explaining Harmony's most basic ideas.
Harmony is a two display (or monitor) application. On your main display (the one with the menu bar) Harmony's main window is displayed. On your secondary display, which is usually a projector, the presentation is displayed. Because Harmony is designed for spontaneity, there is no distinction between editing and presentation modes. This is unlike Microsoft PowerPoint and Apple Keynote. Harmony is always presenting.
If you use display mirroring, in which the same image is displayed on both your main display and your presentation display, Harmony cannot detect your secondary display, and will offer to turn display mirroring off.
After detecting your secondary display, Harmony will automatically scale every presentation element (such as text and media) based on the height and aspect ratio of the presentation display.
If your computer is not connected to a secondary display, Harmony will present in a Preview Window.
When presenting at a church service or other event, Harmony works best being run by a dedicated operator. If you don't have a dedicated operator, look at the Remote and Single Display modes.