Setting Distortion

To the right of the property value sliders in the Motion Settings dialog box, you'll find a sample frame of your clip, labeled Distortion. The sample frame has four corner handles you can drag to distort the clip at each keyframe. Use it to skew, stretch, or even flip the image over.

Distortion doesn't have to result in a wacky effect. It's possible to use it more subtly. Because it can be used on a clip in a superimpose track, it can also be more convenient than using a similar wipe transition.