Just locks probably. Most aircraft have a device they can use to lock the controls into a particular position, the idea is if the aircraft is sat outside it stops the wind banging the controls around. I think they lock the ailerons to an extreme position (full deflection left or right) as its immediately obvious the control locks are engaged. Even if you think this is just where the controls are sitting you'll have to move the yoke to centre before trying to fly, when you do that and it won't move, you know not to start running down the runway. If you lock on centre conceivably a pilot could get in, startup and taxi to the runway without knowing the locks are in. They SHOULD do control checks, that would alert them, but something as critical as the control locks being in you dont want to rely completely on the crew doing that check, as if they forget it or deliberately skip it they could be just starting to fly before they realise something is wrong, at which point they're basically fucked. By setting it up so they HAVE to move the yoke before starting the takeoff roll you pretty much pure guarantee they'll detect the locks being in before the takeoff roll.