What would happen if you rode a roller coaster without a harness? This animation illustrates the likely outcome. This is absolutely not an endorsement to actually try this, because theres a big chance things could go horribly wrong and result in the rider's death.
On some coasters, though, if ones harness was to come loose mid-ride, the rider might be able to survive to do the forces of the coaster speed keeping them in their seat. The animation shows how even on a loop coaster, the rider could potentially stay in ther seat. I've always had a fear that my harness will come un-done when I ride coasters, so this video makes me feel slightly more at ease.
See Also: Toddler has his very own backyard roller coaster