Marvan
my teory is there would be persons who say i do not get constraciton at all, this meaning is maybe they are more aqutic than others..but all human have diaphram and diaphram muscles uses more O2 than other organs if you have any way to directly inject more CO2 into diaphram muscles let says 100ml pure CO2 you will get very strong constractions ,
i here say some people says i do not get any constraction but it means they get but the ratio is very small or their body can easly eliminate in a way, i do not believe there will be any person who don not get cons, it must be imposible, another example some getiing abdominal surgery , the nurse inject C02 into stomach so as to get a good quality filming the area..the person after surgery gets strong constrations 1-2 days go ,
some people living very close to sea shore and some spearos diving very frequantly earning ability and the body not he or she finding a way to eliminate cons or ear equalitions very easly, i mean they are wining by diving and diving
as trux said you should have very very low natural blood acidity, so as to test get deep breath and directly go holding breath while closing your nostril and make your consantration to your stomach, when you close your eyes you can easly make your consantration to any point you desire..i bet you will start geting cons around 3min but will be very soft..next if you make very short hiperventilation(1min) you will the first con stronger it will be like a spark but definetaly will be stronger first test tell us the result lets talk about it later.....all the best..