Link to purport by A C Bhaktivedanta Swami Srila Prabhupada

Transcript of Bhakti-Shastri class on this verse by Chaitanya Charan

How he has heard is described next

Bg 18.75

vyāsa-prasādāc chrutavān
etad guhyam ahaṁ param
yogaṁ yogeśvarāt kṛṣṇāt
sākṣāt kathayataḥ svayam

Synonyms: 

vyāsa–prasādāt — by the mercy of Vyāsadeva; śrutavān — have heard; etat — this; guhyam — confidential; aham— I; param — the supreme; yogam — mysticism; yoga–īśvarāt — from the master of all mysticism; kṛṣṇāt — from Kṛṣṇa; sākṣāt — directly; kathayataḥ — speaking; svayam — personally.

Translation: 

By the mercy of Vyāsa, I have heard these most confidential talks directly from the master of all mysticism, Kṛṣṇa, who was speaking personally to Arjuna.

vyāsa-prasādāc chrutavān, by the mercy of Srila Vyasa Deva I have heard, it was not only vision but the comprehension that was provided. Sanjaya was not only able to see what is happening in battle field but he was also able to understand it so he said etad guhyam ahaṁ param, he is using same word which Krishna has used in 18.64, 

sarva-guhyatamaṁ bhūyaḥ
śṛṇu me paramaṁ vacaḥ
So what is the specialty of this knowledge, it is guhyam param

yogaṁ yogeśvarāt kṛṣṇāt, 

sākṣāt kathayataḥ svayam

kathayataḥ svayam, He has spoken Himself.

yogaṁ yogeśvarāt kṛṣṇāt, that Lord Krishna who is Yogeshwar He has spoken to Himself. How wonderful it is. It is very rare for Lord to appear and it is very rare for Lord to speak philosophy so eloquently and systematically and even rarer is to hear directly so Sanjay has also heard it at the same time which is rare and have good fortune. 

Srila Prabhupada has explained that just as Sanjay has got the message we can also get the message. 

Srila Prabhupada Purport: If one does not come to the disciple succession, he cannot hear Kṛṣṇa; therefore his knowledge is always imperfect, at least as far as understanding Bhagavad-gītā is concerned.

End of transcription.