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 really hard. 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, while the abs are getting tense to apply an opposing force to balance the body.