I never take my knee off the bar on descents and it definitely helps. After years of being fairly stable, my trek became very twitchy, and I'm not sure why. I have experienced the wobble several times. I'm always tinkering with different wheels or tire widths or types or so maybe it was some of that, but we have serious hills around here and the bike was getting scary downhill at speed, even with a knee on the bar. My new bike is heavier but it descends like its on rails and no long makes me nervous on 40 to 45 mph descents.