The fleshy, fibrous part of the diaphragm, known in Czech as "the curtain" (in English it's called a skirt steak) attaches to the lower ribs, sternum, and spine. When you grill it whole and slice it thinly against the grain it releases a distinctive meaty flavor. The name oponka originated during the butchering process; when the butcher breaks down the animal, the diaphragm resembles a curtain due to the small muscle that holds it to the ribs. Not just for grilling, try it in fajitas or stir-fries.