Why is this impressive? Well let's go over a few important aspects of this implementation.

1. It is search engine friendly (SEF) because the HTML is pretty clean, has no Javascript in the body and the content is visible by text only viewers like Lynx or spiders.
2. It does not know the ID's of any slide panels and in fact there don't even need to be any at all. Because it doesn't care, you won't get an error if the element is missing and you also remove the need to explicitly test for it.
3. The HTML is clean enough for a web designer to understand and there is no Javascript to explain or tweak on a per page basis.
4. There can be 1 or 100 sliders or even none and the code will still behave.
5. You don't even a need an ID on any of the elements so you don't need to concern yourself with creating unique ID's.
6. It frees up valuable screen real estate but leaves important information for your visitors to see.
7. You can style the slider via class name or ID.

The Content #2

This one uses an image as the toggle. While you can also define a width, I actually prefer a 100% width clickable area.