how does it work? perhaps it's proprietary that the authors wouldn't want to disclose, but I did not see anything referenced in the site. I wouldn't plug it into mine without knowing g

It isn't proprietary [0]. There is a matrix channel [1] for the project to ask questions.

[0] https://github.com/mCaptcha/mCaptcha

[1] https://matrix.to/#/#mCaptcha:matrix.batsense.net