I tried it a couple days ago and I'm not sure, but I do think it provided some sort of alternate braking.
I was coming down a very steep road in the steep hilly area of Somerset here in Bellevue, WA and decided to hit the DAC button.
I also compared going down part of the road with it off.
With the DAC on, the DAC light flashes on your dash; no real audible sounds loud enough (unfortunately on this test I had the radio on); feet still has to be on the brake and had to still apply a good amount of pressure to prevent accelerating.
With the DAC off, it was my perception that the vehicle accelerated much quicker and a greater brake pressure input was needed to keep the speed down.
So, overall, I think it works...somewhat.