Adds a control panel to the page in the browser. Spacing, colour, and type become sliders. When the values look right, it writes them back into the source file rather than leaving you to transcribe numbers by hand.
Fires on tweak light or tweak max, against slides, websites, or mockups.
It removes the edit, save, reload, squint loop, which is the actual cost of visual tuning. It does not design the page. It only lets you move what is already there.