That's true for Stealth, and I think Intamin's anti-rollback system on Millennium Force is similar. But on the SLC it's a little different. On that one, there is a vertical fin running up the lift hill alongside the chain guide. On top of each car is a pair of spring-loaded cams with teeth on one side. As the train goes up the lift, these cams catch on the fin, which pushes them open so long as the train is moving forward. If the train should move backward, the teeth on the cams will catch on the fin; in fact, the more the car tries to pull backwards, the harder the teeth will grip.
Arrow used a similar arrangement on Big Bad Wolf at Busch Gardens Williamsburg, only they put the cams on the track and used the brake fin.
--Dave Althoff, Jr.