Jersey,
The standard theory for the girth thing is the rubber band theory, which is pretty much what BibBob described.
My personal view is different. I think that working girth can aid length gains because applying a stress in an alternate direction can prevent the structure of the penis building up too much resistence to gaining length. This is also based on structural strength and attempting to alter it to gain.
Its probably worth following the rubber band theory first, cemented girth can’t be undone.