This is an exercise to stretch the quads muscle. Eventually, one should be able to grasp the ankle from behind and pull it into full flexion - but this is useful when full flexion has not yet been achieved. Remember that stretches need to be done slowly and held for a few seconds.
In case you cannot see the video, here is a still sequence -
|Lean lightly against a wall with the foot on a chair.|
|Slowly bend at the hips and lower your body until you can feel the quads taking the stretch. The shoulder, hip and heel should be in a straight line. Hold this position for 10 seconds.|
|Stand up again. Repeat 5 times.|