The rotor of a DC motor rotates with angular velocity that is proportional to the voltage applied to its coils and they have small electrical resistance and therefore would be traveled by intense electric currents if the rotor remained at rest. To change the angular velocity we need to change the voltage applied to the motor and the direction of rotation of the rotor will depend on the asymmetries of the motor and also on the direction of the electric current – reversing the direction of the current the motor will start to turn ‘backwards’. This is how we make a toy train ‘walk backwards’ because we reverse the direction of the current in your rotor.
In turn, when in motion, changes in the magnetic flux over such coils generate a counter-electromotive force (f.c.e.m.), and remove energy from that current and lower the electrical voltages on such coils. The resulting torque will cancel when this f.c.e.m. is equal to the applied electric voltage and the angular velocity becomes constant. In general, by loading the motor when you attach its axis to something that must be moved, its rotation does not vary greatly, but, more power will be required from the power supply (increases the current of the power supply).