Start this pose from a lunge with the front foot flat on the floor, back toes tucked under and the back knee straight. If you use a block under the hamstring of the front leg you can gradually, in your own time, settle into the fullness of this pose. Square the hips and keep the back knee facing the floor.
This pose stretches the hamstrings, rectus femoris, illiacus, and psoas. This pose strengthens quadratus lumborum and the erectors of the spine.
Hamstrings: Substitute with anjaneyasana and make sure you can feel the stretch more in the quadriceps than in the front-leg hamstrings.
Knees: Lightly bend the knees at all times. Work the proper alignment and stay very briefly in the pose.