The ops, I don't believe, have any control over the speed at all. Its all computerized. And the computer averages the last X launches to determine what's working. The sensors at the top of the hill tell the computer if it was too fast, too slow or just right. So in the morning, they run it with empty trains to test, and then slowly add weight. If you vary the weight too much between trains, a rollback will result (as people have noticed when one train has several larger people riding all at once). However, on the next launch, the computer will have made the adjustments.