Janu Sirsasana (Head-of-the-Knee Pose) is a gentle way to stretch the lower back, open the hips and groin, and release the hamstrings. Because it lets you stretch one leg at a time, it's easier than folding forward with two straight legs. For this reason, it's an excellent pose for beginners or those who are less flexible in their hamstrings or lower back.
Here's how to try it:
Sit with your legs straight in front of you and your spine tall. Then bend one knee and place the heel on the inside of your thigh as close to your pubic bone as possible. Exhale and let your bent leg drop toward the floor. Release your "sit bones" into the earth and lengthen your spine, extending up through the crown of your head.
Exhale and turn your body slightly toward the extended leg, then inhale and lift the front torso. Keep breathing and lengthening as you hinge forward from the hips, folding your torso over your extended leg. If you can't reach your foot, use a strap-but don't pull yourself into the forward bend. Allow your body to lengthen forward comfortably, with the lower belly touching the thighs first and the head touching last. Stay for at least five to 10 breaths, then switch sides.