The diaphragm is crucial for breathing and respiration. During inhalation, the diaphragm contracts, thus enlarging the thoracic cavity (the external intercostal muscles also participate in this enlargement). This reduces intra-thoracic pressure: in other words, enlarging the cavity creates suction that draws air into the lungs. When the diaphragm relaxes, air is exhaled by elastic recoil of the lung and the tissues lining the thoracic cavity in conjunction with the abdominal muscles which act as an antagonist paired with the diaphragm's contraction.
It is not responsible for all the breathing related to voice, a common misconception espoused by many teachers but few great singers. One has more control over the abdominals and intercostals than the actual diaphragm, which lacks proprioceptive nerve endings. By training proper posture and balance in the rest of the body, the diaphragm naturally strengthens and works in concert with surrounding structures rather than in isolation.
I muscoli addominali agiscono in un certo senso come antagonisti del diaframma: quando il diaframma si rilassa, durante l'espirazione, i muscoli addominali spingono. Quando il diaframma si contrae, durante l'inspirazione, la cintura addominale si rilassa un po'. Da qui potrebbe nascere la confusione.
Durante la defecazione, quando spingete, fate attenzione a cosa succede nella zona del diaframma.