On tight, downhill switchbacks, I use both brakes, paying close attention to staying just below the limit of adhesion. I also concentrate on keeping the bike as straight upright as possible so I can brake harder without skidding. Doing this well requires paying close attention to weight distribution, what is happening to the bike, lots of modulation of brake lever pressure to get all the braking force possible before the tires start to slip and a pretty high pucker factor.