I suppose they're usually called siyahs (I think from Turkish archery, but I'm not sure). I've made a couple of recurves this way. It's not my favorite but it gets the job done. I simply find a tree truck that has grown in a suitable bend and with straight grain and split it. When dry, shape and attach. I think a dovetail joint is most common, but if you're making a boo-backed bow or similar you can shape the siyah as a wedge to fit between the "bow" and the backing. Wrap with sinew. A benefit is that you can make quite a think recurve this way and thus make it very narrow (it's dicey to steam bend an inch-thick tip to a steep recurve).