After a yogi enters into the samadhi state by doing concentration on some object, I've heard and read that he enters into such a state where all the physical sensations, sound, and sight goes. And his mind becomes totally silent. reference After this I don't understand logically how should he again progress his concentration, or on what object he should concentrate since all his sensations have gone. I am only trying to understand how meditation works logically. I am a little confused with this part.
In samadhi, there is a state of non-duality. When there is "something" to focus on then it is a state of duality. So in the state of samadhi, the object of mediation and meditator are one and the same. Hence there is nothing to focus upon. There is only awareness and bliss.
ता एव सबीजस्समाधिः ॥४६॥ (patanjali yoga sutra)
tā eva sabījas-samādhiḥ ॥46॥
All of these states of consciousness are called sabija samadhi. ||46||
tasyāpi nirodhe sarva-nirodhān-nirbījaḥ samādhiḥ ॥51॥ (PY)
Nirbiija samadhi is attained once even these impressions have become tranquil and when everything has become tranquil. ||51||
From the book of Yogananda Paramahansa
Patanjali defines these states in his classification of the various stages of interiorized meditation. In Yoga Sutras 1:17-18, he refers to two basic categories of samadhi:
(1) samprajnata and
As applied to advanced stages of realization, samprajnata refers to savikalpa ("with difference") samadhi, or divine union in which there remains some distinction between the knower and the known, as in the realization "Thou and I are One." In greater or lesser degree, some modifications of nature remain. But in asamprajnuta samadhi, all differentiations of nature are resolved into the one Spirit. The consciousness of "Thou and I are One" becomes "I am He, who has become this little form of 'I' and all forms." This is not the egotist's proclamation, "I am God!"—the brass crown of megalomania—but rather the full realization of the absolute truth: God is the only Reality. Thus asamprajnata, in its absolute definition, is nirvikalpa ("without difference") samadhi, the highest yoga or union manifested by fully liberated masters or those on the threshold of soul freedom. (...)
Patanjali divides samprajnata samadhi into four stages:
(1) savitarka ("with doubt or conjecture"): intuitive experience mixed with argumentative or doubt-ridden mind;
(2) savichara ("with reasoning or pondering"): intuitive experience mixed with discrimination-guided intellect;
(3) sananda ("with joy"): interiorized intuitive experience interpreted by chitta or joy-permeated feeling; and
(4) sasmita ("with 'I-ness'" or individuality): intuitive experience mixed with a pure sense of being.
These four states, which come after interiorization (pratyahara), are the result of deep concentration (dharana), or superconscious perception as limited to the body. (Yoga Sutras 1:17)
When these four stages of samprajnata have been resolved one by one into the next higher state, the yogi goes beyond them and attains asamprajnata samadhi. This comes in deep meditation (dhyana) in which concentration (dharana) is continuous, with no flicker of interruption; then the object of meditation (i.e., a particular concept or manifestation of God) is experienced as manifested not only in the body but in omnipresence. Beyond these states, in the advanced stages of realization, samprajnata and asamprajnata are understood to mean, respectively, savikalpa and nirvikalpa samadhi. Patanjali says that attainment of the highest samadhi is possible "by profound, devoted meditation on (the Lord) Ishvara (1:23).... His symbol is Aum (1:27)." (Chapter I, God Talks With Arjuna: The Bhagavad Gita by Paramahansa Yogananda)
In Samadhi the mind rests on Supreme Brahma. It no longer operates in the physical or mental plane. The object of meditation has been achieved.
Said the God of Fire: Samadhi denotes a state of mind in which the consciousness, appearing in its spiritual aspect, remains calm and self-contained in itself like a pacified lake or sea and ceases to be operative in the physical and intellectual plains. A yogin, whose mind deeply absorbed in meditation, remains steady and un-flickering like the flame of a lamp kept in a windless chamber or receptacle, is said to have attained to the spiritual state of Samadhi. In this experience a yogin usually loses all his faculties of sense-perception. He hears not, sees not, smells not, feels not when not touched, thinks not, but is inert and inoperative as a log of wood and spiritually rests in the bosom of Supreme Brahma, steady and stationary like the un-flickering lamp light in a windless room.
Miraculous powers and celestial attributes spontaneously develop themselves in a yogin, absorbed in meditating upon the universal soul symbolized by the god Vishnu and thus unmistakably indicate his success in connection with his practice of yoga. The gods wait upon him with the offerings of articles which they themselves enjoy in heaven. The yogin sees, with unaided eyes, unbounded treasures lying concealed in the bowels of the earth and hears the music of celestial spheres swelling in eddies around his person. Kings and rich men seek his favours by the gift of enormous wealth. The Vedas, the books of scriptures and sciences, as well as poetry and celestial alchemy, reveal to him their respective secrets and voluntarily furnish him with all knowledge contained in them. He becomes a healer of malady and a skillful artisan without going through the necessary term of training. Beautiful virgins of paradise hail him with their overtures and unsolicited affections. But a true yogin discards all these as so many trifles and with such man alone the god Visnu is pleased.
Agni Purana 376.1-10