This works really well!
Long story short - it works really well! It's an underlay made of some tacky - not sticky! - material that seems to grip onto the carpet underneath and the rug above. I have a wheelie office chair that runs back and forth over the rug and this product holds the rug almost perfectly in place. It has crept a short distance - maybe 2 cm - over 3 weeks but it would move that much in 2-3 days before, so this is a great improvement.