From what I remember the Momentum button allows you to enter a value of 0-9.
Entering a value of 0 turns off acceleration and deceleration. When a value of 1-9 is entered the Powercab takes that number, multiplies it by a factor of 8 and writes that value to CV3, then writes half that value to CV4. So any values you already have programmed to a loco will be overwritten,
Problems can arise because some sound decoders such as Zimo have specific values in CV3 and CV4 that are neceasary for realistic operation of the sound file.
Loksound decoders have a different problem; acceleration and deceleration values differ from NMRA recommended practices so that their values need to be higher to match other brands of decoders.