The most useful feature on XWord Info is the Finder Page. It’s easy to use for straightforward searches, but there are many deeper capabilities. This video helps you get started.
The tutorial barely scratches the surface of what’s possible. To keep it to 15 minutes, I don’t even mention using $c and $v for consonants and vowels, or how to specify ranges of characters. Click the Quick Instructions button on the Finder Page for examples.
The most important comment in the video might be that you don’t need the video, which the video helpfully explains.
Pro tip: watch it on a computer, and press the full-screen symbol next to where it says vimeo to make the text easier to read.
Update: This later post covers a real-world example step by step — finding words with exactly three Rs, and no consecutive ones.