Another if Shimabuku Tatsuo’s instructors was Motobu Choki.
When he created his Sunsu (SunNuSu) kata, he also paid tribute to this technique of Motobu.
Which was not from the Motobu Naifanchi Kata.
Specifically he shared these photos.
This is how the technique looked in the finished Sunsu kata
Here is Uzeu Angi showing an explanation for the movement.
And the following is yet another possibility.
Among the blog posts about Motobu Choki are the following which eveal more about his teachings.