Unlocking iMovie '09 & '11
There are dozens of reasons you might need to view just a snippet of footage over and over. Editing to music, previewing a transition, or checking the entry of a title can all involve tweaking to get things just right. A way to quickly view just that part of your footage would be golden.
To more quickly view how you are doing, use the "Play Around" feature that is new in version 7.1. Position your playhead right over the point you want to quickly preview, then hit either the "[" key or the "]" key. The left bracket key will preview the two seconds of footage surrounding your playhead, one second before and one second after. The right bracket key will preview six seconds, three on either side.
So make a tweak, hit a bracket key, make another tweak, then hit a bracket key. This is a nice touch.