We all do something for a purpose, to attain desired fruit, and undoubtedly everyone desires Anand (happiness), thus when a living entity realizes Bhagvan who is Sadcidanand svarup, then automatically he experiences Sat (eternal existence), Cit (all divine knowledge), and Ananda (extreme absolute eternal happiness).
Then the person no longer experiences dukha (distress) even though it may appear externally in his life that he might be undergoing great stress but actually he feels not even an ounce of dukha, as once having realized Bhagvan he experiences Ananda solely.
According to Bhagvad Gita (2.72)-
O Partha, the one who so realizes Brahma never gets into delusion, even during the time of death he does not get agitated and thus attains Brahma.
In the same chapter of Bhagvad Gita Bhagvan explains the qualities of such a person who is called Sthitapragya/Sthitaprajna (one fixed in knowledge) from verse 2.55-2.72.
He says that such a person then never experience material happiness or distress arising from various actions, neither does he desires for anything else. He is freed from fear, attachment and anger, even though freed from Maya, he still performs his duty while using his senses only during the time to perform prescribed duty and withdraws them when there is no use.
Now talking about the sudden physical and mental changes that occur during the darshan is hard to explain but still explainable.
First of all a person displays sattvika bhava(S) such as crying, goosebumps, horripilation, shivering etc.
Just like Dhruva maharaj got all the divine knowledge just by the touch of the conchshell of Bhagvan.
Also it is said that he saw the same person who was residing in his heart as Bhagvan in front of his eyes, this also indicated to the statement of Ramcharitmanas ‘Jaaki rahi bhavna jaisi, Prabhu murat dekhi tin taisi’.
So basically Bhagvan assumes the form which a person worships with pure devotion, and appears in front of the sadhak (practitioner).
Having heard from different sadhus (saints), I can also tell that before the appearance of Bhagvan the atmosphere gets filled with the divine unknown sweet aroma, and divine beautiful light fills the space and sweet sounds starts resounding (sounds, aroma differ from Devata to Devata), and therein appears Bhagvan in his own form, he does not put his feet on the ground, neither does he touch any material object,
Even though I have described the best according to my knowledge but still one can get to know fully about the topic only by attaining the darshan of Bhagvan.