The combined practice of the last 3 limbs of yoga (Dharana, Dhanaya and Samadhi) constitute Samyama. These last 3 limbs help us explore internal world through the self knowledge or Svadhayay. By the practice of samyama and svadhayay the yogi gains control over his senses, ego and mind which helps him gain various powers (siddhis) and discover the hidden knowledge of universe (tanmantras).
Dharana is holding the attention on to some particular object internal or external~for example on dwadashanta (12 inch above head) or solar plexus.
Our mind is very lucid and the moment we fix our attention on an object it slips away to another thought. The continuous practice of dharana helps us achieve Dhayana which is the ability to concentrate constantly on an object and keep unbroken attention there for a long period of time.
Samadhi is the third state which is achieved when we get deeper into dhayana and dissolve in the self.
How do we know we have attained samadhi? ~
When the idea of time, space and body vanishes! The more the time, body and space is passed unnoticed the more we are in samadhi.
When the past and present come and stand in one, the mind is said to be in samadhi.
Directing attention to one place, keeping it there for a long time and completely dissolving in that moment, when we are able to achieve all these at the same time that is samyama.
Through the continuous practice of samyama one gains control over siddhis and recieves the infinite knowledge of the mechanisms of the internal and external world.
This takes the yogi from gross to subtler level and it needs to be done slowly in stages. Also, the knowledge of siddhis can free or bind the yogi. To reach the state of mind that is absolute kaivalya and free of karmic impressions, yogi must stay like an innocent seer and not get attached to the power of siddhis.
