It didn't work
It didn't work.  I ran the first screw at random to miss its joist, so I was able to see it looking up when I went down in the cellar.  Then, I could measure precisely where the joists were, went back up stairs, and ran the screws 4" apart over each joist, standing around the tool as advised.... When I was all done... there was no difference in creaking.  Back down in the cellar, I could see that there were no more screws that were visible because they had missed the joists.  Every one of them went into the joists, the plywood is tight on the joists, but the creaking is still there.  Phooey.  Maybe plywood bows a little?  who knows.
Rating: 1 stars