They are special screws that are perforated about an inch below the head. When screwed all the way into the wood, you snap them off with the tool that comes with the kit. The screw ends up being just under the plywood surface and of course totally under the carpet.
I just finished and you can't tell there's anything there. The carpet looks untouched. The kit was $20 at Home Depot. Comes with 50 screws, tips for the screws and a tool to find the studs underneat the carpet and the tool to break the tips off when done. That same tool makes the screw go down to the exact depth needed. Awesome.
No more squeaks. woohoo.