I had the same!! On that moments we had the control over our diapgragm. Therfore we didnt stutter in that situations. Everybody stutters in her or his own way but they have one big thing in common which is that when people stutter on that moment the diapgragm is vibrating. Therefore the method works for every form of stuttering because nobody stammers when they learn to control and retrain their diaphragm to never make this uncontrolled movements anymore (muscle memory of the diaphragm red.) this coordination of the diaphragm is always good and well organised when people sing and as a result the flow of air around the vocal cords is always steady which makes stuttering impossible. Everyone can sing without stuttering. This has a very complex neurological cause but what is does is that the brain automaticallly let the diapgragm move fluently when singing. NoW The stuttering people only need to teach their brains, intercostal muscled and diapgragm to behave the way they do when they sing. You get special breathing excercises at del ferro that come from the world of opera singing in which the diapgragm is the most important muscle after this muscle being ignored so long in medical sciense. Its all about the diapgragm, the second biggest muscle (square based) of the body.