Prayatna Saithilya Ananta Samapatibhyam (II/47)
Prayatna = effort; Saithily = loosen; Ananta = infinite; Samapatibhyam = by fusing.
(While maintaining the posture) Slacken the effort and attune to the 'infinite'.
In the first sutra about asanas, Patanjali asks us to make the posture firm and relax: To make it comfortable. He defines the posture.
In the present Sutra he gives us the technique of making the posture firm yet comfortable.
What is that technique? How to achieve firmness along with relaxation? The technique is very simple.
The first part of the sutra says 'slacken the effort', withdraw the effort. To do that, we have to apply the same technique through which we relax in Savasana. How do we relax in Savasana? Lying on the back of our body we send message (that is carried on through our nerves) to any part of the body. By listening to the voice of the instructor or by auto suggestion we ask, for example, our thighs to relax. As soon as the muscles of the thighs receive that message, they get loosened and relaxed. All the fatigue, tiredness, and tension in the thighs are due to tight muscles. When muscles get loosened, we enjoy relaxation.