You get them going on a nice straight track and they think 'This looks smooth,' and then you dip it down a little to give them a good jolt. Or you have it so that when they go over a hill it looks like they're going to get their heads chopped off at the bottom."
You guide them onto a smooth, even path initially, causing them to think it's going well, and then you subtly alter the trajectory to provide a sudden shock. Alternatively, you design the route so that when they crest a hill, it appears as though their journey will end in a dramatic fashion at the bottom.