I came across this when I accidentally moved my finger on my Magic Mouse when playing back a video.
When a video is playing in Quicktime Player X if you swipe with with one finger on the Magic Mouse (or 2 fingers on the Trackpad) the playback speed can be altered.
Swiping from left to right speeds things up; right to left slows things down.
When you swipe the speed is displayed on the top left of the video and the controls popup is replaced with a simple arrow indicator with gradient lines at 2x, 4x and 8x speed up.
The speed change only happens as long as you have your finger on the surface; as soon as you release the playback resumes to 1x speed.
[crarko adds: When I tried this using the Magic Trackpad (with two fingers) what happened is that is would stop playing the video and scroll to a different spot in the timeline. But as with the Window-throwing hint, I'm willing to believe it's me fat-fingering things. Give it a try and post your results.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110826231424659