There is a lever/button on each of the rails that lock it in place also, or at least I think they do. I know one of them says the word locked on it.
Model X19H Expandable
GMC Sierra Pick up
Been a camper since I was a kid, and that was a long time ago :)
From pup tents with no floors, to tents with floors and flies, to a pop up, to the expandable.