Just out of curiousity, what causes the car to slow dramatically every few seconds when descending a grade with tranny locked in 1 or 2? Is the computer applying brakes, or cutting fuel, or something else? It's a very cool function that really helps control the car on long, steep grades. Me likey. Car is an '07 V6 auto.

 

Same as in drive, coasting to a stop. If you have a wideband or are datalogging/monitoring the O2 sensors, you can see the engine go full lean, like it cuts off fuel, but keeps it enough to stay idling.