Sahasrara is not a chakra as is often thought. Chakras are within the realm of the psyche. Consciousness manifests at different levels according to the chakra that is predominantly active. Sahasrara acts through nothing and yet again, it acts through everything. Sahasrara is beyond the beyond (paratparam) and yet it is right here. Sahasrara is the culmination of the progressive ascension through the different chakras. It is the crown of expanded awareness. The power of the chakras does not reside in the chakras themselves, but in sahasrara. The chakras are only switches. All the potential lies in sahasrara.
When kundalini shakti reaches sahasrara, that is known as union between Shiva and Shakti, as sahasrara is said to be the abode of higher consciousness or Shiva. Union between Shiva and Shakti marks the beginning of a great experience. When this union takes place, the moment of self-realization or samadhi begins. At this point the individual man dies. I don't mean that physical death occurs; it is death of the mundane awareness or individual awareness. It is death of the experience of name and form. At this time you don't remember the 'I', the 'you' or the 'they'. The experience, the experienced and the experiencer are one and the same. The seer, seeing and and seen are merged as a unified whole. In other words, there is no multiple or dual awareness. There is only single awareness.
Raja yoga, kundalini and samadhi :
In the Yoga Sutras of Patanjali you will not come across the word kundalini, as this text does not directly deal with kundalini yoga. However, not every saint, rishi or teacher has referred to kundalini by this name. Kundalini is the subject matter of tantra. When Patanjali wrote the Yoga Sutras 2600 years ago, it was during the period of Buddha and about four centuries before the great era of philosophers. At that time, tantra had a very bad reputation in India because the gifts of kundalini, the siddhis, were being misused for petty purposes and people were being exploited. Therefore, tantra and tantric terminology had to be suppressed, and in order to keep the knowledge alive, an entirely different language had to be adopted.