Think about what happens if you break your arm. Once the bone is immoblized, the pieces fuse back together and, within a couple months, the bone is as solid as it ever was.
A similar phenomenon occurs when vertebrae are fused together in the spine. Some fusions involve plates and screws that are then permanent, while other spine fusions use cadaver bone or bone grafts from the patient to essentially "glue" the area together.
In either scenario, once the fusion has healed, those vertebrae operate from that point forward as one solid structure. There's nothing that anyone is going to do to separate those bones or "break" them loose.
With that being said, when a person with a spine fusion visits our office, we explain this to them and also the fact that we can still work on the segments above and below, because those segments are now bearing an increased load. Also, we can use instruments to adjust the adjacent areas without having to "twist" or "pop" the spine.