Herbs that Invigorate Blood

In invigorating blood, what is the specialty of Yan Hu Suo (corydalis rhizoma)?

Bensky will even go so far as to say that Yan Hu Sun "strongly stops pain".

Source: Bensky, D. (2004). Chinese Herbal Medicine: Materia Medica. Seattle, WA: Eastland Press. pp. 606 [Q893]