This post refers to HoudahSpot 3. The same general ideas apply to HoudahSpot 4. A newer post provides more details.
With HoudahSpot it is easy to find files based on a prototype file: a file that has something in common with the files you are looking for. To find Keynote files: Drag a Keynote file onto a Content type or Content kind criterion row. Drop it anywhere but on a text field. The criterion will update to match the properties of the provided file.
Find by example provides a convenient way to narrow down searches. Drag a file from the current results to a search criterion. You are telling HoudahSpot:
I want files like this one