Abstract: A phenomenon of "robbing the rich and helping the poor" or "robbing the poor and helping the rich" can be found in the blood circulatory system —— the "vascular steal" syndrome, similar to an economic phenomenon: the "Robin Hood effect". There is a scientific biomechanical mechanism in the "vascular steal" phenomenon. This paper gives an exact definition of the vascular steal and its biomechanical mechanism, to help understanding the scientific mechanisms of medical problems from the viewpoint of biomechanics.