The purpose here is to provide a fulcrum around which the knee joint can be stretched. Kneeling on a soft surface (for comfort), place a rolled up towel behind the knee. Rock back over the towel, exhaling as you move backwards. Go as far as comfortable that doesn't impede relaxed breathing. Reduce the pressure on the knee by taking some weight onto the hands, or increase pressure on the knee by sitting back further.
![](https://i.ytimg.com/vi/JBMBrMhnJ8o/maxresdefault.jpg)