Some studies suggest that Arnica works by stimulating the flow of white blood cells which process congested blood, and by dispersing trapped fluid from joints, muscles and bruised tissue.
The anti-inflammatory effects of arnica have also been shown¹ and it is these that may help to reduce pain and swelling as well as assist the healing process.
¹ Lyss G, Schmidt TJ, Merfort I, et al.
Biol. Chemistry 1997 vol 378 pp. 951-961