The switching from relaxation to stimulation is too sudden

When you stimulate yourself with the app, slight desynchronization between the shown start of stimulation and your perception of the current can lead to increased sensation.

This is a purely sensory effect, try not looking at the app while training - the ill feeling will vanish.

There is a sudden fall or increase of current in single electrodes during training

The Nt System is designed to deliver the best comfort during training. This ensures that there is no electrical prickling on the skin.

If the electrode's conductance is suddenly changing during training, the current can change rapidly too.  For instance, if the conductance is below the threshold at the start of a training, the Nt Core keeps the current low. If the conductance rises to acceptable levels, the Nt Core allows stimulation currents of the strength chosen in app. This can lead to a sudden relative increase in stimulation, but does not increase the currents above the values set in the app.

When near the conductance threshold, this can happen multiple times in a training and leads to an unpleasant feeling. Always make sure to stay in warmup until all conductance values are shown to be OK to prevent this issue from occuring. 

Also make sure that the electrodes are wet and tightened to the body for better skin contact.