Just wondering, why when you arm the motors of almost any DJI, it does this double spin and the slowse the revs? Is it because of propeller tightening? Or is there som other reason?

I think I saw this on another thread and I want to say that it is for the prop attachment and a calibration. My P4P has done it from the beginning also.

Its checking if the props are properly attached. And if it is using the folding props like the spark/mavic its using the centrifugal force to make the props straighten out before flight. The props will fly off or the motors will stop if it notices the props are not properly attached