2.50 It may be external, internal, or midway, regulated by time, place, or number, and of brief or long duration.
2.51 Energy-control which goes beyond the sphere of external and internal is the fourth level- the vital.
2.52 In this way, that which covers the light is destroyed.
2.53 Thus the mind becomes fit for concentration.
2.54 When the mind maintains awareness, yet does not mingle with the senses, nor the senses with sense impressions, then self-awareness blossoms.
2.55 In this way comes mastery over the senses.
So here we are at the end of Book II. Book II focused on the physical and moral practices of Yoga, how to go about it, and what we can obtain by it. The fundamentals are always something that we will need to come back to from time to time. The farther you go down the path, the greater you will be established in the basics. When these basics become ingrained, the Yogi moves on the the esoteric portion of Yoga in Book three.
Most practitioners get involved with Yoga at either a superficial level, or go straight to the esoteric level. Book two gets mixed up somewhere in between. Do not think that there is anything wrong with wanting to jump into the deep end, but untill you learn a how to tread water, you will drown in a sea of confusion.
I jumped straight into the deep end. My path was the path of knowledge, which lead me to data overload. Drowning in my own confusion, I swam to the shallow end of the pool and worked my way back to the understanding I was trying to achieve from the get go. When I read Autobiography of a Yogi, I wanted to have everything that Yogananda did. I thought that Yoga would give me the ability to move stuff with my mind and ascend to higher levels on consciousness. I found out it does not work that way.
Credit for this picture goes to Swamij.com. One of the best sources on Yoga.
These last five Sutras talk about mastery of the body and it's energy, called Prana. With mental and physical purity, the foundation for deep meditation is laid. If you have started meditating, you should notice how distracting it can be. I started out by using foam ear plugs to block out the noise around me. Ever-time my dog barked it was like a stab in the ear. My dog must have the loudest bark on the planet. I have heard him from a block away with all the doors and windows closed.
Notice that Sutra 2.51 and 2.52 speak about going to the fourth level and the destruction of the darkness that covers the light. The fourth level is that which lies beyond the thought and absolute stillness of the mind. Words cannot describe because words are just symbolism put into verbal and written form. It goes beyond abstract concepts such as our awareness of space, time and even the Yoga Sutras themselves.
I hope that you have found my commentary on Book II useful and hope that Book III is beneficial as well. Hopefully it wont' take me another year to finish.