Very thin pieces can be heated in very hot water and bent in a form - chair slats for example. Boiling too long can reduce the properties of the lignin. Mostly people use a wood like ash to steam bend. It has excellent bending properties. Some woods don't bend at all well.
Wood is composed of cellulose fibers and lignin which is the glue that holds it together.
Wood cells are rather like long square honeycombs. When steamed the lignin softens and the cells can move a little. The outside must stretch, the inside compress, the center remains pretty much the same. Some woods can't take the strain and compression. Woods like ash and oak have very open grain and this helps the steaming.