The field is uniformly perpendicular to the cylindrical rotor surface and does not change polarity over the 360-degree perimeter of the cylindrical part of the rotor. That means if the field is considered entering the rotor between the slip-rings it must leave the rotor though the rotor's end surfaces. It is not important that the magnetic field is generated from the stator; a similar field may be generated from the rotor with the same effect (the magnetic fields would not rotate with the rotor).
