General ⇒ Multi-line TOC entries

[rspringuel]

How does LaTeX determine when a TOC (and LOT and LOF) entry needs to be broken onto multiple lines? I know that \@tocrmarg controls the width for multiple line entries, but it seems only very loosely related to determining when a line qualifies as multiple lines. I.e. increasing it a bit doesn't always force a long line to split but increasing it a lot will.