If I understood correctly, one needs to suck with diaphragm, by inhaling. Is it so? I mean, I suck through straw using my mouth only, I can breathe through nose

If I understood correctly, one needs to suck with diaphragm, by inhaling. Is it so? I mean, I suck through straw using my mouth only, I can breathe through nose while sucking into mouth. It seems for me that it wouldn't work. Am I right?