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If you have tight hamstrings, using a belt or strap for reclining big toe pose (supta padangusthasana) and you will experience a wonderful stretch in your hamstrings and calves. Many people are flexible enough to reach their big toes while lying flat on their backs with one leg extended, without letting the back come up from the floor, but just as many people are not that flexible.
Here's the fix:
Start by lying flat on your back with your legs straight out ahead of you and your feet flexed. Bend your left leg and bring your knee towards your chest. Wrap the yoga strap around your left foot and slowly extend your left leg so it is perpendicular towards the floor. Hold the strap in both hands and pull on it gently, to ease left leg towards your chest. Don't bounce, jerk, or pull too hard, and remember to extend through the back of the knee of the left leg. Carefully release the leg to the floor and repeat for the other leg. If you are frugal, you don't have to buy an official yoga strap. An old necktie works fine, as does a bathrobe belt or a rolled-up towel.