The Difference Matte effect creates a transparency a mask or a matte by comparing the pixels in a source layer with the pixel in a defined difference layer, and then keying out (eliminating) pixels in the source layer that match both the position and color in the difference layer.

In this video tutorial, Creative Cow Leader Aharon Rabinowitz shows you how to simulate airflow over an object by using the After Effects CS3 puppet tools with a particle system.