I have cut this description of the study. Please click on the title for the full abstract.

De-qi, not psychological factors, determines the therapeutic efficacy of acupuncture treatment for primary dysmenorrhea.
Xiong J, Liu F, Zhang MM, Wang W, Huang GY. Chin J Integr Med. 2011 Oct 12.

[Epub ahead of print] Institute of Integrated Traditional and Western Medicine, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, 430030, China.
OBJECTIVE:
To study the impact of De-qi (, obtaining qi) and psychological factors on the efficacy of acupuncture treatment for primary dysmenorrhea, with an attempt to explore the relationship among De-qi, psychological factors, and clinical efficacy.
RESULTS:
Complete data were obtained from 120 patients, 60 patients in each group. There were statistically significant differences in pain intensity (W=2410.0, P<0.01) and pain duration (W=3181.0, P<0.01) between the two groups. The number of De-qi acupoints (W=1150.5, P<0.01) and the average intensity of De-qi (W=1141.0, P<0.01) were significantly higher in the manipulation group as compared with their non-manipulation counterparts. The correlation coefficients between De-qi and therapeutic efficacy of acupuncture were greater than those between psychological factors and therapeutic efficacy.
CONCLUSIONS:
Compared with the psychological factors, De-qi contributed more to the pain-relieving effect of acupuncture in subjects with primary dysmenorrhea. Moreover, manual manipulation is a prerequisite for eliciting and enhancing the De-qi sensations, and De-qi is critical for achieving therapeutic effects.

If we try to figure out why a De Qi sensation would be key to effectiveness (my teachers from Shanghai insisted that it wasn’t, btw) maybe it’s because acupuncture mimics an insect bite or some other insult encountered in nature that would start a healing cascade. Just thinking, thinking… Trying to come up with a mental model.
More on the practice here.