The series goes from feet to head, as this is the flow that releases prana the most naturally. Spiritual energy is udana prana and it rises to head, so doing the series lessens physical tensions that are obstructing the spiritual prana. On a circulatory level, blood moving back to the heart (venous return) is to be stimulated first before arterial flow. This is because we want to promote elimination before improving quality of input. Progressive relaxation can be done either way, starting from the head is best as we want to drain excess mental energy and increase bodily attention ending at the feet has a grounding effect.