Most of these sounds cut off quickly to increase the effect. The "M" forces you to close your mouth cutting off the sound in a similar manner. The "R" is presu

Most of these sounds cut off quickly to increase the effect. The "M" forces you to close your mouth cutting off the sound in a similar manner. The "R" is presumably because these sound also tend to have a rolling sound to them (reverb?). Think brrr or purr. You could probably say something like PREEEB with similar effect though.