A seat belt fracture (also known as a Chance fracture) occurs during a car accident. Usually this happens when the pelvis is anchored by a lap-strap without a shoulder harness.
When the driver slams on the brakes or is hit from behind, the victim is thrown forwards against the belt. The vertebral body may be split and severely displaced away from the rest of the bone.
Ligaments holding the spine together are also torn or damaged. A severe enough injury can cause damage to the spinal cord, even resulting in paralysis or death.