Watch the Motion of the Thumb online guitar lesson by Muriel Anderson from 50 Right-Hand Techniques You Must Know

Moving the whole arm adds power and drive to the thumb. Most of the time you will be doing a free stroke in which the thumb swings in front of the fingers. You can also do a rest stroke with the thumb, pushing the thumb down with the arm so that it ends up resting on the next string.

This thumb rest stroke brings out the bass notes in the same way that a rest stroke with the fingers brings out the melody notes.