Its a
threaded delrin insert. The idea is that the drip tip sits tight, allowing you to screw the insert down into the body. This opens up airflow
through the holes in the top.
The delrin insert can be very tight out of the factory. I had to get some WD40 down into the
threads to get it moving the first time. If you do this make sure you remove the insert completely, clean off the WD40 and apply a small amount of lubricant to the threads. I used a tiny bit of Vaseline.