The app (by means of the built-in smartphone magnetic field sensor) determines the number of rotations of the magnet and, therefore, the number of coil turns.
To do this, you need to put the smartphone at the selected place (where measurements will be taken), make sure that there is no magnet nearby, and press the TOP "0" button.
The app is designed to calculate the number of turns when winding an inductor (inductance coil).
It is also possible to determine the rotation of a smartphone without using the magnet (e.g., when winding a large coil).
To reset the number of turns, press the DOWN "0" button TWO TIMES.