These small rear-facing steps alone are responsible for most of the extraordinary performance I have been measuring since 2003 and Dick Johnson confirmed in 2006.
Recent measurements indicate that the very high performance values and corresponding sensitivity and lack of consistency are due to the upper surface rear-facing step being located a little too close to the leading edge. I anticipate that moving it aft slightly will compromise some performance for consistency and lead to another useful performance improvement. Meanwhile, a lower surface only configuration appears to offer about 15% without messing up the polar.
The article gives an overview of the modification, installation instructions, measurements that led to this mod and a non-mathematical explanation of how it works, IMHO.
To read the article, go to http://www.deturbulator.org/TheDeturbulatorTape.pdf.