Extended triangle pose is a common Hatha posture with many variations. Standing at the top of your mat, step back with one foot. The feet should be about one leg length apart with heels in line. The front foot is pointing forward and the back foot is angled out anywhere from 45-90 degrees. Bring your arms up to parallel with the floor. Reach forward with the front arm, stretching forward through the torso. Lower the straight torso over the front leg and keep the body in one plane. In other words, the torso should be above the front thigh. Rest your front hand on the shin or a block, or take a toe grab (as in the picture below). The back arm reaches up toward the ceiling. Roll the rib cage upward, opening up the chest. Keep the legs straight, with knees soft and not locked.
Think about opposing forces in this pose. The arms are moving in opposite directions, one extending downward and one upward. The torso extends in both directions, with the tailbone and crown of the head moving away from one another. The feet are rooting down, while the hips are reaching up.
Some of the physical benefits of this posture include stretching through the groin, hamstrings, calves, and spine; opening through the shoulder complex and chest; lengthening and stretching in the neck; and toning in the core and leg muscles.