This[1] video does an excellent job of explaining the mechanic with visuals and code snippets.
Carmack's public post[2] on the "bug" of strafe-jumping is endearing in light of how much it influenced game design in the following decades.
[1] https://www.youtube.com/watch?v=rTsXO6Zicls
[2] https://www.gamers.org/pub/archives/plans/johnc@idsoftware.c...