I had tried teaching this at one point, and it was unsuccessful. It's when the bird hangs upside down on a perch and then climbs back up, sort of like a flip around a perch. It seems easy to train, just requires targeting... but what could I have been doing wrong? Does this trick require a special kind of perch? Should it be a thick perch?
It basically looks like this:
(Sorry for it being dark, the video isn't mine)




