In the meantime, it may help to look at the Google or ImageMagick descriptions of the WebP encoding settings to see what they do:
https://developers.google.com/speed/webp/docs/cwebp
https://imagemagick.org/script/webp.php
There are also tooltips in Spine, hover over the WebP sliders and other controls on Spine's export dialog (press F1 to make the tooltips appear more quickly).
The defaults are pretty good most of the time. You may want to adjust Quality and Alpha quality, and possibly Compression. A number of the settings don't usually make much difference, but we provide them because they do exist and it's possible that someone needs such fine control, even if most users don't.
We still need to publish proper documentation, of course!
