In the text Tripura Rahasya, King Janaka expounds to sage Ashtavakra about the nature of samadhi.   He says that nirvikalpa samadhi is a continuous experience of every being. It is only interrupted by thoughts and objects.   When the mind is extroverted one doesn’t realize the nirvalkpa samadhi that is constantly there, as the attention is busy grasping this and that.    The purification of the mind is with the introverting of the mind, from outside to within. As the mind purifies the samadhi becomes apparent and eventually one can effortlessly stay like that (effortless samadhi).