Hello my dear DJI. I have one question - what is it button on the stick for diving?

I don't own this item myself, so can't say from my own experience, but from the looks and placement of it, I would suspect it is some kind of a quick-release lock. I could imagine that pressing that down and rotating the mount part would let it loose and you could quickly switch it to another mount solution with a similar connector. And, then quickly placing that back on the mount and twisting would automatically lock it back to it's place.

Hi there. When this buckle is pressed, it will release the upper part.

What measures need to be taken to remove part of the transport?