For the advanced stages of kundalini yoga when the trunk of the subtle body is being infused, it becomes necessary to use the backward neck lock as compared to preliminary practice where the kundalini passages through the spine into the brain and does not spread through the trunk.
While the front neck lock which is a drawback of the chin towards the throat, facilities, guides and restricts the aroused kundalini so that it remains centered in the sushumna nadi central passage and is better controlled once it enters the head of the subtle body, the backwards neck lock entices kundalini to spread through the trunk banishing heavy astral energy and dark zones in the trunk. The value of this is that the yogi’s subtle form become refined and allows for reach into the chit akash during the meditation that follows.
I was informed by Yogeshwarananda that currently due to the numerous distractions in this digital age, it is near impossible for anyone to reach chit akash steadily and with sure focus. The requirements for yoga meditation given by Patanjali, will hardly play out because the disturbances now are steady and seem to be necessarily even for serious ascetics.
He proposed that a yogi should take titbits of chit akash in meditation and collect those in each session and be satisfied with that, rather than be idealistic and not get anything. If one cannot get a full meal in one sitting, then one may take whatever little one gets here and there, and accumulate that. Why stick to an idea which will never happen because the circumstances just will not permit it.
For chit akash reach, one must clear the entire subtle body of heavy astral energy. That is one thing that cannot be avoided. Once there is regular infusion of the subtle body, every part of it, then it is likely that one will reach chit akash even if it is only fragments of it.
During meditation when one sees tiny spots of light behind the mental darkness or interspaced in it, one should accept those spots as particles of chit akash, one should welcome that energy and pull that into the core self. The same pulling action should be used with naad sound resonance, which should be regarded as the sound of chit akash.