[quote name='LewisK1975' timestamp='1485951382' post='3227922']
When above the strings, it's a thumb rest, for finger playing.
When below the strings, it's a tug bar, for thumb playing.
That's the general way it's explained, I think! YMMV!
[/quote]
Spot on.. Though as a thumb rest it sort of locks you into a single playing position... I seem to recall Leo assumed that bass players would primarily want to use their thumbs...