How to find the bass line from the soprano line?

Ever wondered how to take a hymn tune, place it in the right hand part, and at the same time to be able to play the improvised bass line which would go smoothly with it? This skill would allow you to create perfect simple biciniums and in fact would help you in your hymn playing (especially if you are a poor sight-reader). Why is it so and how to do it? Watch this video to find out this trick.