At a given speed, the bars will start to wobble uncontrollably. Braking makes things worse - much worse. All you can do is hang on for grim death using sheer strength to keep the bars in line. Slide back off the saddle to push the centre of gravity back from over the front wheel. Hope and pray that you slow down sufficiently to regain control before a corner.
Prevention is difficult as it's caused by the frame shaking.
This may be because the frame's too big or too light for your weight. A loose headset can bring it on too. End of horror story. With care, preparation and good technique, downhilling on tour is a fantastic experience. Without going into the mountains even, it's good to have confidence on the downhills as this is where a weaker rider may gain time against a stronger partner and reduce the need for continuous stopping.