G'day and welcome Robert.
In a word, nothing - the CF is simply pressed between the rotating spool and the 'fixed' drag plate (like the old TLD5, 10 or 15).
I considered gluing the CF to the spool or notching a groove in the lip of the spool for a spring clip but, having tested exhaustively and fished the reel, don't believe drag performance would be significantly improved by either.
That said, if I were redesigning the reel from scratch, a tabbed CF washer on either side (as per an Accurate TwinDrag) would doubtless be preferable.
PS Probably worth bearing in mind that I'm no expert, just an enthusiast...