When I first was experimenting with WindWorks, I overdid the “aperture corners” inward horizontally thing and would wind up with what felt like an extreme pucker at times that would not have a good sound or ability to articulate, etc. even though I often was successful in obtaining the pitch.

After a while, I developed a better sense for the “aperture corners”; it is not the corners of your mouth, but is a subtle movement of the muscles surrounding your lips (I believe). And the movement is subtle, I have spent a lot of time experimenting with using as little engagement as possible to change pitch and have been amazed at how little it can take.

Hope that helps.