You have probably seen this gadget called Squeeek-No-More in the store or at home show demos and wondered if it actually works. It does, under certain conditions.
The idea is to drive a screw into the floor, perhaps right through the rug, have the threads hold in both the floor joist and in the sub-floor, and then break off the head to leave no sign of the operation. It works best when there is one piece of plywood over the joists. More layers of wood and this idea is not so efficient. It is also important to realise that this screw will not pull the plywood down tightly to the floor joist, but will simply lock it into place where it is. Hence it is critical that you put your weight right over the problem to force the plywood down to the joist before driving the screw. Then the screw will hold it down tight.
You must use the special driver and the special screws that come with the kit. The driver stops the screw at exactly the right place. The screw has a pre-fractured point on the shaft that allows the head and the shaft to break off just inside the plywood, leaving the screw threads in the right place.
If you have a Berber carpet, one that if you pull a thread it can run right across the room, you must wrap each screw with scotch tape so that the screw will not grab any threads and destroy your carpet.
On simple one layer floors it can do a good job of tying down a squeaking floor.