I would like to know how to mask (hide with a blur or mosaic) a part of a video. The part(s) that I need to hide will usually be in motion. I guess it is the same effect that is used on "Cops" and other shows to hide license plates and people's faces.
Place an addtional event of your video clip in another track right above the current track, making sure they're in sync with each other. Apply the blurring/mosaic effect you wish to the upper track, and also apply the cookie cutter to this track. Using the cookie cutter you can select a small portion of this blurred track to be shown superimposed upon the unblurred track. Use keyframes to adjust the position of the blurred area as often as necessary to keep up with the motion.