The old seatbelt design worked on a ball bearing system. When the front end of the car dips (when in a crash for instance) the bearing rolls forward and locks the mechanism. It takes a lot to get them to lock when you just pull on them when stationary. Try tugging on them when doing an emergency stop, you'll find they'll lock easily. I've been thinking of getting some later generation Celica/MR2 ones that are proper inertia reel ones, but need to get the old one off first to see how they bolt to the car and the design of the anchor. I really don't feel safe knowing that the belt won't lock immediately, there's too high a chance that your head will hit something first.