Many of us have wondered what that mysterious toggle button is at the top right corner of the Table of Contents view (looks like a grey chain-link).

I asked enTourage, and here's the explanation.

When selected (white background, the chain-link appears connected), then when you navigate through your eBook, the Table of Contents will stay in sync with your position - i.e., the Reader and the TOC position are linked. If you go to the start of the book, the TOC will reposition itself to the entries at the start of the book.

When deselected (dark background, the chain-link appears unlinked), then when you navigate through your eBook, the Table of Contents position will not move, but continue to show the same section of the TOC you were looking at - i.e., the Reader and the TOC position are unlinked. If you go to the start of the book, the TOC will not reposition itself to the entries at the start of the book.

The Linked mode is handy if you want to see any annotation entries, or TOC entries, around the page you're currently sitting on, i.e., the TOC will show the context of the page.

The Unlinked mode is handy if you're actually focussing on the TOC to review a specific section of the book and want to skip around in the book arbitrarily, but want the same part of the TOC staying visible so you can quickly jump to another TOC entry where your primary focus is, i.e., you can skip to the front of the book, or to the index at the back, or do a Search, and the TOC will stay put.

Now that we know the function of this icon, no doubt it will come in handy in some situations