What this is actually doing.
This is the focused version of “Composition rule visualiser”: do the one job, inspect the result, and move on without opening a full editor.
Presentation & Layout
See where the focal point sits.
Move a focal point over a rule-of-thirds guide to judge the composition.
Rule-of-thirds preview
The thirds grid is a starting framework, not a rule that every composition must obey.
Useful to know
Four things worth knowing.
Where it stops.
It solves the named job, not every problem around it.
What to inspect.
View the slide full-screen. Editing view can hide how crowded a layout feels when presented.
Worth knowing.
Keep the original until the finished result has worked in the place you made it for.