At the top: Tom Lehman uses his right knee to stabilize his backswing
U.S. Ryder Cup captain Tom Lehman, whom I've been coaching since 1989, has an underappreciated record in Ryder Cup singles play. He's 3-0 in singles and has lost only four holes in those matches. One reason he was so tough under Ryder Cup pressure is the transition move he developed to start the downswing.
Tom grew up in Minnesota, where he had to play frequently in cold and wind. He developed a fast-rolling, low hook for distance. That served him well through college—he was a three-time All-American at the University of Minnesota. But that hook action wasn't reliable on the PGA Tour. He learned to use his legs to synchronize the squaring of the clubface at impact with his arms and the rotation of his body. After he rotates around a stable right knee going back, the initial movement of his left knee toward the target helps him monitor his rhythm under pressure.
If you're having trouble keeping your swing consistent when a match is on the line, key on your left knee to start your downswing. When you practice, remember to support your backswing with your right leg, then move your left knee and thigh laterally, then rotationally, to your left. This allows your arms to drop the club inside the target line on the downswing for long and consistent shots.