The Idea of the purge is to remove the oxygen completely from the atmosphere. I was taught that purging from the bottom slowly fills the part with argon from the lowest point, forcing all the oxygen out the top or vent. Reversing the purge allows the argon to fall from the top through the air inside the component, causing it to mix with the oxygen, creating a contaminating environment, but eventually becoming pure argon. Both systems work, however purging from the bottom removes the contaminants faster. Sometimes there is no choice which way to do it, depending on the access to the joint, fixtures and what not.