The word acintya ("inconceivable") is very significant in this connection. God's energy is beyond our conception, beyond our thinking jurisdiction, and is therefore called inconceivable (acintya). Who can argue this point? He pervades this material world and yet is beyond it. We cannot comprehend even this material world, which is insignificant compared to the spiritual world—so how can we comprehend what is beyond? Acintya means that which is beyond this material world, that which our argument, logic and philosophical speculation cannot touch, that which is inconceivable. Therefore intelligent persons, avoiding useless argument and speculation, should accept what is stated in scriptures like the Vedas, Bhagavad-gītā and Śrīmad-Bhāgavatam and follow the principles they set down. This will lead one to understanding.
(Bhagavad-Gita----8:9----purport).

Kṛṣṇa, antar bahiḥ, He is situated outside and inside. Simply we have to make our eyes and senses perfect to see Him. Kṛṣṇa is not invisible. He is visible. But one who has got the eyes to see Him, one can see; otherwise not. Therefore in the Brahma-saṁhitā it is stated, premāñjana-cchurita bhakti-vilocanena santaḥ hṛdayeṣu sadaiva vilokayanti (Bs. 5.38). Those who have developed Kṛṣṇa consciousness, love of Godhead, they can see the Supreme Personality of Godhead, śyāma-sundaraṁ acintya-guṇa svarūpam, the reservoir of all transcendental qualities. Acintya-guṇa... Acintya means inconceivable. The qualities which Kṛṣṇa has got, that is inconceivable by us. He has got qualities.
(Lecture on Bhagavad-Gita----13:1-2).

Therefore Lord's power, exhibition of power and sharp nails, they're all inconceivable. Śrīla Jīva Goswami has explained, "Unless you accept inconceivable power of the Supreme Lord, there is no understanding." Acintya. Acintya-śakti. Acintya means inconceivable. You cannot speculate how it is happening, how in the lotus flower there is so hard nail that immediately, within a second, it could kill a great demon like Hiranyakasipu. Therefore it is acintya. We cannot conceive. Acintya. And therefore Vedic instruction is acintyā khalu ye bhāvā na taṁś tarkeṇa yo jayet: "Do not apply your poor logic in the matters which is inconceivable by you." There is no logic that how lotus flower can grow nails. They say, "Mythology." Because they cannot conceive within their poor brain, they cannot accommodate how things happen like that, they say, "Mythology." Not mythology. It is fact, but it is inconceivable by you or us. It is not possible.
(Lecture on Srimad Bhagavatam----7:9:6).

This Kṛṣṇa is called Śyāmasundara. Śyāma means blackish, but He's very beautiful, very, very beautiful. Śyāmasundaram acintya-guṇa-svarūpaṁ. Acintya means inconceivable. His qualities, His transcendental qualities, are inconceivable. When in the Vedic literature it is said that God is quality-less, nirguṇa, nirguṇa means He is transcendental to these material qualities. But His transcendental qualities are there. So He is called that acintya-guṇa-svarūpaṁ govindam.
(Lecture on Srimad Bhagavatam----7:9:9).