In my 2003 Four-Runner, I don't understand under what conditions I should engage the DAC switch. I get that it is for downhill assist, but under what circumstances should I use it? Towing a travel trailer, I can't tell that there is any effect so I go ahead and downshift the transmission to lower gears. I engaged in 4-low on a steep downhill grade and got an alarming loud clunking noise from the brakes and immediately shut it off. So when should I use it?

If you don't mind, please give me some technical information as to what happens when it is engaged. Apparently it works with the anti-lock braking system.

Thanks.