OK, just to be sure we are all clear on the concept: when you are barrelling down a curving hill you turn by leaning not steering (except in ice & snow) so your handlebar light continues to light the area straight ahead, where your wheel isn't going to be in a nanosecond. It lags behind the turn. Your helmet light is aimed where you look, which is into the curve and beyond so that you can see where you will be going. The reason I run both lights is to give the constant straight ahead warning to traffic while still being able to check the side of the road and approaching intersections. It's like running both a constant taillight and a blinkie, motion may be more noticeable, but the constant light gives more info to the driver of your location. Pardon the long response, I'm still in the on-line course mode of answering essay questions.