If he seriously understood, then he will act on ātmā. And otherwise, simply understanding, that is impersonal understanding, Brahman only understanding. Simply theoretical knowledge that "I am Brahman," that's all.
Puṣṭa Kṛṣṇa: One who has such knowledge... What is the position of one who has such theoretical knowledge but doesn't apply it?
Prabhupāda: Not theoretical. Practical.
Puṣṭa Kṛṣṇa: What happens if someone doesn't apply it practically? What is their position? Are they higher than the person in ignorance?
Prabhupāda: No, what do you mean by theoretical knowledge?
Puṣṭa Kṛṣṇa: Just like so many... In Indian philosophy everyone knows that there is ātmā, but still, people continue to act on the gross bodily platform for sense enjoyment.
Prabhupāda: Then, if he seriously understood, then he will act on ātmā. And otherwise, simply understanding, that is impersonal understanding, Brahman only understanding. But what is after that, Brahman knowledge, that they do not know. So they are almost as good as the animals. Brahma-bhūtaḥ prasannātmā (BG 18.54). Therefore they are not happy. Simply theoretical knowledge that "I am Brahman," that's all.
Indian man (1): They know the knowledge but they don't act according to that.
Prabhupāda: Therefore they are not happy. Otherwise one who knows "I am Brahman," brahma-bhūtaḥ prasannātmā—he will be happy always. That has become fashion. "Table chair brahma-jñāna," sitting on the table-chair, smoking and talking of Brahman. "Armchair brahma-jñāna." Although Kṛṣṇa is giving information, brahma-bhūtaḥ prasannātmā na śocati na kāṅkṣati, samaḥ sarveṣu... mad-bhaktiṁ labhate param (BG 18.54), so they do not make further progress, mad-bhaktiṁ labhate param, to achieve that platform of bhakti. Therefore it is as good as no knowledge.