Back EMF, it's the way the decoder senses how the loco motor is running. It is a CV value and can be changed by the user on a programming track providing that the user's DCC system allows CV programming.
I know coreless motors, like the one in the DJM class 71, don't work well with the chip's BEMF sensing enabled.
Can't remember what CV it is, but it's been discussed earlier in the topic, a search should find it.