There is a positive side (the red) and a negative side (the black) of the current flow. It is very common for a user to feel the current on one side more than the other. Interestingly, although the positive side is clinically proven to be slightly more effective, which is why some users like to practice switching sides or “polarity switching”, there is no rule or consistent reporting that the positive side is felt more. Some users feel the positive side more and others feel the negative side more. This is normal and okay.