Kaytieeldr is correct - if you keep the seat belt buckle facing up as normal, then tighten the belt, as it gets tighter it will get closer to the back side of the car seat. When you need to unbuckle it, you won't have enough clearance between the buckle and the car seat back to actually lift the buckle. If you flip the buckle over, this won't happen as there will be a natural space behind the car seat because the lower part of the plane seat back curves away slightly. It's hard to explain, but if you've ever made the mistake of not flipping the belt and then tightened it a lot, you'd understand PERFECTLY.