In some cases during the training, you can feel something like a stimulation in muscles, where there is no stimulation set.
This is because of two possible effects:
First, the applied current can spread over neighboured muscles, when the user is sweating a lot. This creates a feeling of stimulation in areas, where there is no electrode or the electrode is set to 0 (zero).
Second, if muscles of a certain area are stimulated, opposing muscles are tensed to create a force against this muscle contraction. You can easily recreate this, by applying a reasonable high stimulation all over the back, and feel the abs are getting tense to apply an opposing force to balance the body.