When skating downhill, a tense back causes you to arch your spine and lean forward, compromising the ready position (shoulders over hips over heels) that makes for effective speed control.
When you keep your wheels directly under your hips as you turn (rather than pushing one skate outside your ready position line), your body weight adds to the pressure where the carving skate meets the street. This results in increased friction to maintain control and a manageable speed. A solid ready position makes such control possible for skaters at all levels.
By consciously reminding yourself to relax your lower back, you can quickly regain your ready position on a hill, plus the leverage that results in a powerful, speed-scrubbing edge set. Once your lower back is loose, you can tuck your hips back in above your heels. If you don't know how to relax your back, try thinking about tightening up your abdominal muscles slightly, which also pulls your hips in.
Try focusing on a relaxed lower back the next time you hit that long hill. You'll find it a valuable tool. And if you're a skier, you can expect a magical advantage next ski season after a summer of concentrating on slalom turns with a relaxed back. Your new mobility in the moguls will surprise you!