This position calls for deep flexibility of the hip flexors and flexors of the spine. Moderate hamstrings flexibility of the supporting leg is called upon as well. The pose can be helped by having flexible shoulder extensors.
Due to so many areas of concentration, many people find this pose rather challenging. The reason being incorrect training. The most common way to train this pose is forcing the body into it. This is often achieved by assuming a position closely resembling it and pull as hard as possible for as long as possible, hoping for the results to come one day.
This application avoids the pain of the stretch reflex, allowing for much quicker and safer flexibility gains. Having lengthened each tissue by itself, the pose is put back together. Most people see amazing results just after the first time of trying this program.
Have a look at the muscles animation video for Natarajasana:
You can take a quick look at the contents of the program here: