It's titled Conditioning for Greater Leg Extension by Mary Virginia Wilmerding.
I was searching for anything on strengthening the psoas/hip flexors. I found this doing a search on how ballet folks develop the strength to hold up their legs high in the air.
Here is an excerpt:
"The second group was taught a five-minute therapeutic exercise and asked to perform it in three sets of ten repetitions, on both legs, three days a week for six weeks. The exercise was a modified leg raise: in a long sit position the dancers leaned back on their hands and the leg to be exercised was turned out and the knee flexed, like a front attitude (see Figure 3). The aim was to engage the quadriceps as little as possible and focus the work on the hip flexors; the dancers were coached to achieve this aim. In six weeks we brought everyone back and re-measured their active and passive ranges of motion. Again, we measured the difference in the two photographs. All the dancers who did the therapeutic exercise saw an increase in their développé à la seconde. The average height increase was 6.5". The dancers that did no extra training showed no change in the six-week period."
I have been training this for a week now and I notice I don't engage my huge quadriceps as much anymore when I lift my leg. Look at the average height increase over six weeks according to the article!
Here is the PDF to take a better look at it. It includes pictures:
http://www.iadms.org/associations/2991/files/info/Bulletin_for_Teachers_1-1_pp5-6_Wilmerding.pdf
The reason I searched for this is to increase my kicking ability.
I also notice that my posture is better right after I work on my hip flexors. I must have weak/tight hip flexors.

