How could the spine bend after it's had fusions?

It doesn't. The goal of fusion is to get the bone of 2 adjacent vertebrae to grow together and thus stop the motion at that segment. The segments above and below will take up some of the motion and you will be stiffer. The reality, though, is that the majority of your bending motion comes from the hips rather than the spine and most patients don't notice a big difference after a 1 to 3 level fusion.
Assuming the fusions. Are healed, you move through the unfused regions of the spine and at the hips as well where the head joins the neck.