BEMF & High Frequency Operation
DCC decoders pulse (turn on and off) the power to the loco motor - the longer the pulses, the faster the motor runs. Full voltage is applied when the power is on.
Early decoders did this at a low frequency (well in the range of human hearing) which made the motor jerk at the drive frequency and harmonics of it. Many would sing or jerk. Manufacturers provided drive frequency adjustments to minimize these issues.
Then came high frequency drive which operates the decoder at the upper limit of human hearing and well beyond any mechanical resonant frequencies. (Called Silent Running, Quiet Drive, SuperSonic, etc. by the manufacturers.) It fixed most of the problems listed above. However, a new problem entered - reduced torque at low throttle settings. TCS introduced Dither to counteract this negative effect of Quiet Drive - sort of like taking one drug to counteract the adverse effects of another! Other manufacturers have copied the idea and given it their own names, but it is some form of pulse timing adjustment.
BEMF Decoders
Back EMF (called BEMF) sensing decoders use the motor's intrinsic ability to function as a generator to sense what speed the motor is running. When you are running at less than full speed, the decoder only has the power turned on to the motor for a percentage of time (roughly the percentage of full speed that you are running). The decoder senses the BEMF generated at lower speeds and adjusts the following pulse length to compensate for changes in the motor speed. Note that nothing in this relates in any way to how fast the pulses are being applied (i. e. whether the decoder is normal or high frequency - with or without Dither). The response time and memory of the BEMF system determines whether it is an "auto goose" to help the loco past binding mechanism or tight track work issues (short time constant) or "cruise control" to keep the train running at the same speed.
One BEMF item which is NOT frequently discussed is the number of bits of resolution. Lenz, as an example, uses a 12 bit system, which will allow the decoder to sense speed variations as small as 0.02%. Other manufacturers may be as coarse as 8 bits (0.3%). The more bits, the smoother the control.