Hoptroff watches contain a magnetometer that can determine magnetic North. In the No.15, it is used to point the direction to Mecca. In other Hoptroff watches, it is used to make the seconds hand point to magnetic north.
The watch needs to be held horizontally for the magnetometer to function correctly. As with all magnetic compasses, large metal structures such as buildings can distort magnetic field lines. You can check this by moving a few meters and seeing if the direction indication has changed significantly.