Here is an except from masterviews.com
a) The simplest way to do this is to use the "Hidden Slide" feature available when you are in Slide Sorter mode.
After the slides that need to be linked to the "optional" reference content, add the slide with the extra content, but after having selected it, "hide" it by selecting the appropriate button in the toolbar.
When you are running your presentation the "hidden" slides are not shown unless you press the "H" key while you are on a slide that precedes a hidden slide. So, when you need to call up the "extra" slide, you press "H". After having used it the presentation will resume back to the next following slide.
b) A more sophisticated approach to this is the one of using invisible links on the pages that need have a link to optional content.
What to do?
1. Create a rectangle as large as the whole slide.
2. Make the rectangle unfilled (no color inside) and delete the border (set line color to "no line").
3. While the rectangle is still selected go to the Slide Show menu and select Hyperlink.
4. Now select the PowerPoint presentation file to be linked to. (You have the option of specifying to which slide to link inside the new presentation.)
Done.
Now while you are delivering this presentation you can simply click anywhere on those slides equipped by the invisible rectangle and easily jump to the other presentation set.
To come back to the original presentation you can adopt the same technique. On selected slides apply invisible rectangles that allow you to link back to your original presentation (and even to the specific slide you want).
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