Progressive disclosure defers advanced or rarely used features to a secondary view, making applications easier to learn and less error-prone.
Progressive disclosure is a powerful tool in interaction design, but it's often used precariously.
Scrolling itself is an interaction and scrolling to reveal content is a universally accessible version of progressive disclosure. Scrolling is often more intuitive, easier, and more effective that clicking small click targets like links and buttons.
scrolling > clicking
Further reading: http://www.nngroup.com/articles/progressive-disclosure/
The time required to rapidly move to a target area is a function of the distance to the target and the size of the target. Targets located near screen hotspots, such as the corners and edges, are easier to target with a pointing device. This is because the cursor stops at the edge. This is why Mac OSX places the menu bar and dock at the edges of the screen.
Further reading: http://en.wikipedia.org/wiki/Fitts's_law