This intense forward bend is Krounchasana (Heron Posture), an extension of Triang Mukhaekapada Paschimottanasana from the Primary series. Krounchasana stretches the hamstrings and glutes of the straight leg while lengthening the quadriceps of the bent leg. The abdominals, psoas, and spinal extensors are strengthened, as are the muscles of the legs if eccentrically engaged to press against the arms.
To enter the posture from Dandasana, bend your right leg back so that thigh bones are parallel and the ankle is straight. Lift the left leg (bend the knee if you need to, but keep it as straight as possible) and hold the foot or bind the hands around the foot as shown. Inhale as you extend the leg, arms, and spine to look to the foot (pictured below). Exhale as you pull the leg close and sit up as straight as you can. Use the lats to draw the shoulders down and lift the belly in. Stay for 5-10 breaths and repeat on the opposite side.
When proficiency in the posture is gained, it is an interesting challenge to jump directly into the prep position from Adho Mukha Svanasana without allowing the straight leg to touch down. If the glutes are tight, there is a tendency for the straight leg to swing open. Keep the leg forward and the hips level. Resist the temptation to round the spine with steady engagement of the core stabilizers and strong lift of the bandhas.