Specifically I’m thinking about the harness that pulls down over your head, like the one on Shock Wave and The Demon at Six Flags Great America. You pull it down over your head, and it locks until the ride is over. What is going on inside there? What guarentees the thing won’t open up while your upside down. What is the internal mechanism that prevents it from opening?
I just came back from Paramount’s Kings Dominion, and even though I didn’t have the opportunity to take a rollercoaster car apart while I was there, I got the sense that the mechanism is a simple claw and ratchet design. That drawing may not help much, but picture the right side of the joint locked in place. The left side can then only rotate in one direction (to make the restraint tighter). When the ride comes to a stop, the right side of the joint is pulled away (ka-chunk!) and the left side can rotate away from the rider.
Several of the rides had chains or straps that hooked on to the restraint bar, in case the ratchet failed I guess.