This pose gives a good release for the lower back, and is a great opener for the muscles around your ribs. For endurance athletes, the second can have a huge impact. You'll be able to breathe more deeply, and use up less energy than when you're fighting against tight muscles to expand your ribs.
How-to: Lie down on your back, arms and legs straight. Lift one knee into your chest. Hold either the knee or outside of your foot with your opposite hand, and roll the knee across your body to the floor. Reach your opposite arm out perpendicular to your body.
Tips: If the backs of your legs are tight, keep your knee bent close to your chest, holding the outside of that foot, or resting your forearm across the knee to provide some weight. Breathe deeply, allowing your body to lift with each breath in, and relax deeper into the twist with each breath out.
Benefits: Releases muscles along the spine and ribs.