That is a insulator to keep the center pin from shorting out against the 510 threaded side. If you look at the threads of the 510 connector there will be 2 air holes right above the threads. That insulator need to be inserted into the center of the connector and pushed up until it is above those air holes, otherwise it blocks the airflow when you attach it to your battery. If you need to you can pull out that center pin (it is just held in by that insulator and sometimes another one at the top right under the coil) put in the insulator and push the center pin back through.
I got the center pin out and put the insulator in and I don't have enough strength to put the pin back through. Should I just cut it so that it is above those air holes since i can't do it the other way?
Even if I could push it back through I don't think the insulator would sit below those air holes. A lot of compression would have to happen to make that be the case.
We were just talking on another thread today how awesome ECFers are.