Hand-to-Big-Toe (Padangusthasana) is a posture takes Standing Forward Fold (Uttanasana), one step further. In addition to merely folding forward from the hips, this pose utilizes the hands to deepen the posture. The fingers clasp around the big toes to help the torso press closer to the thighs.
In addition to stretching the hamstrings, lower back, and calves, it will also stretch the shoulders.
This is a great posture for warming up the back side of the body for deeper forward bending postures later in the practice.