my full carbon fork came with a cheap (they are really just a couple of cartridge bearings and a spacer) NECO headset. it has one of those. the outside of it is knurled and digs into the softer carbon, sort of like a star nut. once installed it isn't removed easily. here's a pic of something similar.



if slide over and up the plug before installation it will expand as the plug is tightened, but if all is well, well... i guess it isn't necessary.