A barber generally keeps on snipping the scissors in between the actual cuts. There is a particular phrase for this behavior. I have come across this phrase before, but can't recollect it now. The phrase was a generic term for such involuntary acts and the barber's one was a good example for it. The closest I can some up with now is "fidgeting", but I don't think that was the word. Two close phrases I came across today are "muscle memory" and "automaticity"
How about "idling the scissors"? "Idling" is typically used in conjunction with cars or motors for running without a load, so while it is not the professional term, it's pretty idiomatic. If it is supposed to free the scissors from cut hair, "clearing the scissors" would probably work.
I think the actual motivation is that the screw in professional scissors is self-adjusting, so if you cut a number of thick strands in succession, it might become too loose for good cutting. Clattering the scissors "without load" would serve to readjust the tension.
So another word that might match the purpose is "priming" the scissors.
Tics, perhaps? (This is often part of 'nervous tics', though not all tics are nervous ones.)
It sounds like a complex motor tic.
(Edited to clarify "nervous" and link to definition.)
It sounds like this action is similar to dry firing a weapon.
Dry firing is done for a variety of reasons. Often, it is done for the purpose of maintaining a movement without making a flinch.
In the case of a barber, they're dry snipping the scissors in order to maintain their rhythm and muscle memory for the action.
However, you may also be misinterpreting their actions. They're often point cutting which is a means of thinning out the ends of the hair or snipping away strays after making their main cuts.