Seatbelts on attractions in Disney have a "fail safe" design, meaning that it requires power to be applied to *unlock* the seatbelt. In its resting state, there is a physical latch inside the buckle that prevents the button from being pressed down. To unlock, power must be applied to this latch to move it out of the way and allow the button to unlock the belt when the Guest presses it.
That's the design for the buckles on rides like Star Tours and Tower of Terror. I presume the latches that are built into the ride vehicles (Soarin, Test Track) are of similar design.
I don't know for sure, but they may also have physically designed in an extra safety feature where the power to release the seatbelts can only be applied when the ride vehicle is in a safe position in the station. Coasters and other thrill rides by major manufacturers have been doing this for years. Power used to lock/unlock a lapbar is applied through electrical contacts on the side of the ride vehicle. When the coaster train leaves the station there is no physical connection. The only way to unlock the lapbars would be to return to the station, or the ride operators can carry out a battery pack of some kind that connects to the train to power the latches. I could easily see Soarin designed in such a way that the carriages have to be in the down position to be connected to the circuit to unlock. (Don't know for sure, just speculating)
As for the seatbelts themselves that trip a safety sensor if they're cut or too much slack is reeled in during the ride (like if the rider pulled out too much seatbelt on Tower and hid it from the CM to leave a lot of slack), I've heard talk about such sensors second- and third-hand but I have no direct knowledge of such sensors.
-Rob