PowerDot applies electrical impulses to contract or twitch your muscles. The electricity always flows between two points, with one point (or channel) being between rectangular pad and one round pad and the second channel is between rectangular pad and the second round pad.
Therefore, if you disconnect round pads, you won't be able to feel any muscle contraction at all.
The following factors can be attributed to the difference in your feelings:
- Positioning of the pads. If rectangular pad is located closer to the nerve or muscle motor point, the brain can be putting more priority in delivering one muscle contraction feedback signal over another. Feel free to experiment with the position of round pads on a muscle in order to see if you can find a position closer to the muscle belly (where motor points are usually located).
- Muscle fatigue. If muscle fibers under your round pads are more tired than the ones under rectangular pad, a higher intensity will be required in order to notice the difference.
In general, as long as you can see contractions and twitches under any of the pads, there is absolutely no reason to worry, as your muscle as always contracted along its full length, between rectangular each of the round pads.