Sitting meditation is to see our minds. It isn't about entering any specific samadhi as there are no samadhis to enter.
In four modes of birth and nine realms, every thought is samsara. There's not a moment that isn't rebirth. There's not a place to let our minds be attached to. Hence, we must rediscover our minds if we wish to part from samsara.
Chan cannot be understood through theories. As the teaching goes, "where it may be, the mind ceases". Wherever the mind is, there'd be no arising. It is to return to the mind. Do not cling or fixate on anything. Exert in your practice instead of getting entangled in theories. Knowledge alone doesn't help us to realize Chan. Be free from languages and symbols. Right where we are is where the mind no longer arises. Do not let it wander around in samsara. Then, we must know a way. What would that be? The way could purify our spirituality so it is freshening, stable, and spacious. Elevate our spirituality in such a manner.
The purpose of sitting meditation is to see our original face. As Buddha Shakyamuni said, "I have the treasury of the true Dharma eye, the marvelous mind of Nirvana, the subtle dharma gate of the true form of the formless." Notionlessness being the truth. Through notionlessness, we can realize the truth. By knowing the truth, we can fill all the gaps among differences.
What is the original face? What are we before parents give us life before the physical body is formed? This body is none other than the product of karma. This is not our original face, nor is it in the samsaric phenomena. As long as we are caught in phenomena, we cannot see the truth. Not knowing the truth, we'd always be occupied with dualistic ideas and perceptions. Not parting from dualism, our minds could fabricate everything. With notions, there comes the mind.
When we sit for meditation, we are not doing so with any minds. Do not think about the past, the present, or the future over and over again when meditating. The three times are ungraspable. What should we do if the past, present, and future don't exist? Well, "Buddha-nature is constantly purifying and clearing. Where could there be dust?" There's nothing substantial from the beginning, not in the past, not in the present, and not in the future. As the three minds are unattainable, this is the original face. It is as it is. So, we shouldn't get caught up or confused when meditating. Be sure to untie all the knots and entanglements in the state of ungraspable.