The idea is simple and can be found integrated into some sites (vk.com, habrahabr.ru) and Opera Mini. Whenever you're reading a page deep down you'd probably want to quickly get to the page top (e.g. while reading Facebook News Feed). Most likely you'll have to use scroll wheel or aim a scroll bar. The easier option is to use a pop-up button (which pops at the right moment) to quickly scroll the page up. That's what the plug-in does.
1. Move mouse pointer to the left and see the button pop up (whenever cursor gets to 200px area the button is showed):
2. Click the button:
3. Have the page scrolled up:
Clicking the button again (which is renamed to "DOWN") will scroll the page back to the position where "UP" was clicked.
The plug-in can be downloaded here.
Extension contents can be found here.