If the seat belt is taught then it's unlocked, if it's not, then you're somewhere between "clicks". The B&M restraints always leave me with about two-three inches of "wobble" because I'm between the first click and the next one. The seem to have a large gap between one-click and the rest.
I've had conversations with UC about this and I can never remember the exact details. I'm pretty sure though that once the train leaves the station the "closed mechanism ratchet" is in operation. So the the restraints can get tighter and lock, but once they're in the "ratchet zone", they're locked.
The train can leave the station with the restraint open though and not "far enough down".
Somebody will know exactly how the locking mechanism works, but I'm pretty sure it's a simple system. When the train enters the station, there's that large runner that lifts and that pulls the "pin" that unlocks the restraint. When you leave, the runner pushes down and puts the restraint lock in place (or the ratchet lock mechanism). From that point, there's nothing to suddenly activate or deactivate the locking mechanism (except a key to unlock a restraint manually).
I think - please correct and add detail!
As for scary rides. Shot towers. I hate the anticipation because I hate the feeling of the airtime they give. They height doesn't help, but I find heights less and less worrying as time goes on.
Expedition GE Force made me nervous. It was the first "big ride" I'd really done since the US though, and there's something about the way the coaster suddenly dives off and away into the woods so far - it looks and feels much higher and more exposed than it is and there's slightly more anticipation than you get on say, Bizarro, because the track at the top just vanishes down into the trees and twists away.
Other than that, rides that swing "naturally" when up high. I don't like being high when unpredictable things are going on (so Star fliers, simple kids elevator rides, etc).