A great source for Yoga Postures by Mark Giubarelli.
YOGA POSTURES ASANA YOGA POSES YOGA POSITIONS TADASANA

 Parivrrta Janu Sirsasana IV

Translation: Revolved Knee to the Head 4

From a seated position extend both legs diagonally out.

Lean towards one hip and rotate the other inward (medially) bending the knee. Lean so it is easy to grab the foot and re-position the knee.

At this point the knee will be too far inwards and it has to be opened out. Once this is done rotate the body sideways more fully and bring the shoulder down to the extended leg. This will stretch the hamstrings and side of the body.

Bend the elbow and grab the big toe or foot. Look Upwards and lengthen through the top of the head and heel.

Take the other arm behind you and grab the foot.

It takes a moment for the hamstring muscles to relax but when they do lengthen behind the knee so that the ends of the hamstrings are stretched and the top of the calf muscle is stretched.

Benefits
This posture increases hamstring, quadricep, anterior shoulder and leg adductor flexibility.Turn the leading leg outward and the Iliotibial band stretches well.

Tips Use the elbow and outer arm to aid in the twist. Pushing it into the inner leg on the extended leg will deepen the twist.

Vinyasa Yoga postures to transition from and to:

Ardha Kapotasana Paschimottasana Hanumanasana Janu Sirsasana Parivrtta Janu Sirsasana Janu Sirsasana II Parivrtta Janu Sirsasana II Parivrtta Janu Sirsasana III Parivrtta Upavistha Konasana Parsva Upavistha Konasana Akarna Dhanurasana Ardha Bhujangasana

Yoga Teacher Tips

This posture can be a nice lead into or out of Hanumanasana .