The name comes from 'yakhni', the Persian-derived word for a meat stock, because the rice is cooked in the meat's own broth instead of being layered with raw marinated meat like kacchi.
Beef or goat is cooked down with potatoes and whole spices, then the rice, often a short-grain aromatic variety such as chinigura, is cooked in the same pot so it absorbs the meat's spiced broth. It's a Chittagong wedding and festival staple, softer than the kacchi biryani style favored in Dhaka.