The trim brakes slow the train down enough to prevent it over speeding onto the second lift (into the Crypt building). Over speeding causes the ride to shut itself down as a safety precaution.
*tongue-in-cheek* A genius move during the design phase, I think you'll agree
Edit >> RyanYoxall - I had to laugh! You're right, trims are used to reduce the forces on riders, but on Th13teen? Any less force and I think we'd all go backwards