This is done as an add on to an Downward Facing Dog
So you are in your down dog position and you will lift one leg off the floor keeping the leg straight.
At this point the hips are still facing the floor.
You then bend the lifted leg and bring the heel close to your buttocks.
As you do this you will open out the hips and that will open out the chest slightly but you must keep both hands in contact with the floor.
Look under the armpit of the opposite arm and try to look at the bottom of the foot of bent raised leg.
You should feel a great stretch through the front of the hip and its a great shoulder/arm strengthener.