Lets talk about selecting areas. We'll cover:

Selecting an area from the screen
By dragging
By hovering over a window

Selecting is easy. I'll use the simple mode UI.

First; click the icon that has an image of a 'crop' rectangle on it. That enters selection mode.

You can select by clicking and dragging the handles, and you can move the selection around just by dragging it.

If you want to clear it, hit the C key. Use the number keys (top of keyboard) to change to a certain percentage of the screen size. These are useful to get close to a size, which can then be fine tuned to an exact size (say, recording a video frame from a browser)

There's also a quick way to select a window. Just hover the mouse over the window, and press W. This will make the selection take on the size of the window under the mouse.


Show Selected Areas
iShowU V6 can show your selections. Open up settings for V6, and go to the General tab.

Note: the selection rectangles are never recorded!

The video explains it better :)