Back EMF stands for Back Electro Motive Force.
Back EMF is also sometimes referred to as BEMF, Load Compensation, Feedback or Speed stabilization by various manufacturers.
Back EMF is a means by which a decoder can monitor a motor and keep it turning at the same speed.
Back EMF can give much better control of motors, especially with poorer examples such as Lima where running is normally tremendously improved.
Back EMF can give a 'cruise control' effect, keeping speed constant as a train climbs or descends gradients and negotiates curves.