The --tolerance option doesn't work as expected (or: documentation needs to be improved)

[horeah]

I am unable to use the --tolerance option in a meaningful way, as the mapping of the specified value to the actual shape of the tube is completely opaque (to my mind at least).

Here's a capture of a tube created with --tolerance=0.05:

Can you please explain a bit more how the --tolerance works? The short mention in the help is hardly informative enough...