This is a neat little lick that I picked up from looking over Pancho and Lefty.
The really interesting thing to learn about here is how Willie did a great job of using almost the same lick, along with a couple of slides, to move himself over three different areas of the guitar neck. All of these areas kind of have their own characteristics, and can lend themselves to different ideas and phrases, so having a way to go between them can be a nice tool to keep in the belt.