How do these work (Jet Fighter Engine fins)

On these. When they accelerate the fins (silver slat things) around the engine close in. But i never knew how they did this.

How do they close the fins?

It's a convergent duct so it will produce thrust. As air goes through a convergent duct, it speeds UP and the pressure goes DOWN. I'm pretty sure they use hydraulics to make the duct convergent and divergent(opposite of convergent).

If it's a supersonic then the principle is different.