Swami Satchidanandendra Saraswati (SSS) was a great Sanskrit scholar who made an extensive study of Shankara’s writings and commentaries and subsequently wrote many books on Advaita Vedanta. According to SSS, there are essentially 2 methods to ‘attain liberation’ outlined by Sri Shankara:
1. Firstly, in those who are ripe, merely hearing (sravana) a teaching equating oneself with Brahman, will result in liberation. For some who are slighlty less ripe, some repeated contemplation (manana) upon this teaching will be required too. (Tom’s addition: A ripe mind may be a mind that is rendered extremely pure and subtle by long and sustained spiritual practice, or a mind may be ripe due to other more mysterious factors including ‘God’s Grace’.)
2. Secondly, for those who are not able to ‘attain liberation’ merely by hearing +/- contemplating a teaching such as ‘you are that’, one must also undergo prolonged meditation (nididyasana) which will directly result in liberation.
We can see that Bhagavan Sri Ramana Maharshi broadly states the same here.
SSS in his text the Theory of Vedanta writes on p. 153:
‘In addition to Karma and Upasana, there is a kind of concentrated contemplation called the Adhyatma-Yoga which leads to immediate intuition [of Brahman, ie. Self Realisation]’
You can see that SSS is stating that this Adhyatma-Yoga is not classified as ‘karma’ or action, and is also not classed as Upasana (meditation on objects), but is something else that is a direct means of knowledge or liberation.
SSS writes in ‘The Method of Vedanta’ p. 147:
‘The aim of the one practising sustained meditation (nididhyasana) is different [to Upasana, defined here as meditation on forms/objects]. He tries to attain direct vision of reality (here in this very world) by turning his mind away from all else [ie. all objects]. And there is the difference — as against upasana — that after the rise of knowledge nothing further remains to be done. It is this sustained meditation that is referred to at Kathha Upanishad I.ii.12 by the name ‘Adhyatma Yoga’. In the Gita it is sometimes called ’Dhyana Yoga’ (e.g. XVI11.52). In the Mandukya Karikas it is called ’restraint of the mind’ (G.K.III.41, etc.). Its nature is described there in that latter work. Everywhere its result is described in the same way as right metaphysical knowledge, and from this comes immediate liberation (sadyo-mukti).’
You can see that SSS defines Upasana as meditation upon objects, and that this is considered to be action or karma (and so will not directly lead to realisation), whereas nididhyasana is a special type of meditation which involves turning away from objects, and this type of meditation is not considered to be ‘karma’ or action, but a direct means to knowledge (as karma pertains to objects only, not to the actionless subject).
Here again this is stated more clearly in the introduction to the text Adhyatma Yoga:
‘The subject dealt with here viz. Adhyatma Yoga, also known as Dhyana Yoga, Mano-nigraha Yoga, Samadhi Yoga and Nidhidhyasana, is treated these days as a Kartru Tantra Sadana. But in the Shankara Bhashya throughout, this Adhyatma Yogi or Dhyana Yoga is treated as a Vastu Tantra Sadhana’
Kartru Tantra Sadana means action, which being limited, will therefore not lead to something unlimited (ie. liberation or the Self). Vastu Tantra Sadhana means something that will lead directly to the Supreme Truth (Vastu), ie. that which is a means of Knowledge or Liberation.
In summary, SSS writes on p 143 of The Method of Vedanta’:
‘the highest kind of candidate is able to acquire immediate intuitive vision that his Self is the Absolute from merely hearing the relevant upanishadic texts once. These people who realize the goal by merely hearing the texts once have nothing further to do…
‘…But those who are not able to acquire intuitive knowledge of the meaning of the texts in their own direct experience have to go on hearing the texts and reflecting over them to remove the doubts that prevent their meaning being understood, and they have to continue with this until intuitive knowledge arises. For we see that those of dull understanding acquire knowledge through diligent repetition…
‘…But those who cannot acquire intuitive knowledge of reality by hearing and reflection alone have to resort to sustained meditation also. In any case, the general rule is that hearing and the rest have to be continued until there is intuitive knowledge of reality. For attainment of intuitive knowledge of reality is their purpose’
To see how SSS and Shankara define meditation or nididhyasana, see here: What is Vedantic Meditation?