Looking up at the base of the lift hill of an SLC. It's not easy to see, but there is a vertical steel fin on the far side of the chain guide. The two green prox switches are located between the chain guide and the fin in question.
This is one of the anti-rollback cams mounted on top of the train. There is a set of these for each car. The springs hold the teeth closed, while the motion of the anti-rollback fin pushes them open.
Back to the Ride Tech and Safety index...