Move Along Path

im trying to get a movie clip to move along a square path but running into some problems. ive been searching around and so far can only find this http://proto.layer51.com/d.aspx?f=952 on senocular about doing such a task. now this is written in as2 and ive been trying to convert it to as3 but i dont really understand whats going on mainly the return this; in the Path class (what is it returning exactly?). if anyone knows of another way to do this in as3 or could help me understand whats going on in that senocular post in a as3 way would be great.

thanks!