Actually, if power is lost, I believe the ride carriages lock into place where they are.
On the ride console there are two buttons that can be used to stop the show. The one prominently labeled "Emergency Stop" operates the same way that an E-Stop button does on all heavy machinery, namely it stops all machinery where it is and locks it into place, just like you'd have in a factory assembly line or something like that. CM's operating the attraction are taught to only use this in extreme cases where any movement by the carriages could put someone in danger (like a few years ago where a guy and his kid somehow got out of the carriage before takeoff and the CM's missed it, and they were suddenly seen standing there after the carriages were in the air). If the E-stop is pressed, only Maintenance can release the locked system, so riders would be stranded in mid-air for a while.
The other stop button that CM's are told to use to stop the show is labeled something like "Return to Home" and it does just that, it immediately stops the show and lowers the carriages to their home position. This is obviously the preferred option if the emergency is with one of the riders up in the air. If something is wrong with someone up there, you want to bring the carriages down right away, not lock it up in the air.
In case of a power loss, I presume that they function as if the E-stop had been pressed.
-Rob