Darshan implies being in the (or feeling the) presence of God (or saint). The power of darshan is extraordinary and can transform the heart.
Here is what happened to S.S. Cohen upon the first darshan with Sri Ramana Maharishi.
I was alone in the Hall with him. Joy and peace suffused my being - such a delightful feeling of purity and well-being at the mere proximity of a man, I never had before. My mind was already in deep contemplation of him - him not as flesh, although that was exquisitely formed and featured, but as an unsubstantial principle which could make itself so profoundly felt despite the handicap of a heavy material vehicle.
The Sun by its very nature gives out light, similarly the great masters by the nature of their realization give out energy, this energy can be received by the seekers. Just as how the earth receives sunlight from the Sun, but a cloud can cover this light from the sun, similarly the student can receive the energy from the Guru, but some unwanted karmas and tendencies prevent this from happening. That is why not all get the same result from Darshan. thus purification is important.
But this does not necessarily happen to everybody. Not everybody has the same impact upon darshan of a yogi, a saint, an avatar or God. Different types of devotees get different results for darshan.
What happens internally?
According to this book "What is Hinduism?" by From the Editors of Hinduism Today:
Darshan is the vibration that emanates from the illumined soul as a result of his inner attainment, be he a yogi, pandit, swami, guru or a rishi.
Darshan from a great soul, like the pollen of the flowers, can stimulate healthy sneezing and cleansing if one's subconscious happens to be congested.
The guru's darshan lifts repressed subconscious patterns that have been out of the flow of the cosmic patter of regenerative life.
Basically darshan is like entering a strongly flowing river or immersing oneself in the ocean. Just as how the flow of water cleanses the body of all impurities, the flow of energy/vibrations from the Lord/Sadhu floods the mind of the sadhaka and removes all the unnecessary tendencies, negative feelings, etc.
Sakamya Bhakti (Devotion with desires)
These are the bhakta's who feel that seeing God (or having His darshan) should bring them some benefit (as if seeing Him is not a benefit in itself!).
Sri Krishna says in Bhagavad Gita 7:16 that there are four such devotees:
O best among the Bharatas, four kinds of pious men begin to render devotional service unto Me—the distressed, the desirer of wealth, the inquisitive, and he who is searching for knowledge of the Absolute.
Thus the result of having His darshan is according to the desire that the devotee has.
- The distressed experiences great healing, their diseases are reduced or even removed!
- The desirer of wealth get wealth, they obtain the necessary wealth and property or whatever it is that they seek.
- The inquisitive get their doubts clarified and they get mental satisfaction.
- The one who seeks knowledge of the Absolute, gets it :).
How much of disease gets removed, how many doubts get clarified, how much wealth is obtained, how much knowledge of Self is obtained, depends on various factors such as karma of the individual, level of realization/devotion of the devotee and ofcourse God's Grace :)
But this is all only if you want something from that Darshan. What if you want nothing at all in return for Darshan of The One.
Nishkamya Bhakti (Devotion without any conditions)
These bhaktas desire nothing from the Lord. Having darshan is verily their purpose, journey and goal. Start is darshan, middle is darshan and end is also darshan.
When you already have what you want (being in God's presence), what else does one require? This is their attitude.
An example of such a devotee is Sri Chaitanya Mahaprabhu:
Sri Chaitanya Deva says in Sri Sikshastakam Text 8:
ashlishya va pada-ratam pinashtu mam
adarshanan marma-hatam karotu va
yatha tatha va vidadhatu lampato
mat-prana-nathas tu sa eva naparah
I know no one but Krishna as my Lord, and He shall remain so even if He handles me roughly by His embrace or makes me brokenhearted by not being present before me. He is completely free to do anything and everything, for He is always my worshipful Lord unconditionally.
There are no desires and no conditions in this devotion, thus Darshan is constant and always uninterrupted with no requirement of any kind of good or bad results.
Sri Swami Sivananda mentions in Bhakti Yoga:
Whatever you want the Lord will certainly give you, if your Bhakti is intense and if your prayers are sincerely offered from the bottom of your heart. But you will not get supreme satisfaction, immortality and Moksha through Sakamya Bhakti.
Aspire for Nishkamya Bhakti. Your heart will be purified and the Divine Grace will descend upon you. Be in communion with the Lord, you will become one with the Lord and you will enjoy all the Divine Aisvaryas (Divine attributes like wisdom, renunciation, power, etc.). All the Vibhutis (Special forms in which the Lord manifests) of the Lord He will give you. He will give you Darsan. He will help you to dwell in Him. At the same time He will give you all the Divine Aisvaryas also.
In other words, the one who desires nothing from God, but God Himself, that devotee of the Lord is already liberated, already realized and already in Truth. Because he is in Supreme Bliss, he does not need anything else.
For such bhakta's every moment is darshan. In fact having association with such bhakta's is darshan for other devotees! All the best.