Nanda Maharaja understood this exchange of love (between mother Yasoda and Krsna, and her binding Him to a wooden mortar), and therefore he smiled and released Krsna - from wooden mortar

Expressions researched:
"Nanda Mahārāja understood this exchange of love, and therefore he smiled and released Kṛṣṇa"

Srimad-Bhagavatam

SB Canto 10.1 to 10.13

Nanda Mahārāja was surprised that Yaśodā, Kṛṣṇa's mother, could have bound her beloved child in such a way. Kṛṣṇa was exchanging love with her. How then could she have been so cruel as to bind Him to the wooden mortar? Nanda Mahārāja understood this exchange of love, and therefore he smiled and released Kṛṣṇa. In other words, as Kṛṣṇa, the Supreme Personality of Godhead, binds a living entity in fruitive activities, He binds mother Yaśodā and Nanda Mahārāja in parental affection. This is His pastime.

SB 10.11.6, Translation and Purport

When Nanda Mahārāja saw his own son bound with ropes to the wooden mortar and dragging it, he smiled and released Kṛṣṇa from His bonds.

Nanda Mahārāja was surprised that Yaśodā, Kṛṣṇa's mother, could have bound her beloved child in such a way. Kṛṣṇa was exchanging love with her. How then could she have been so cruel as to bind Him to the wooden mortar? Nanda Mahārāja understood this exchange of love, and therefore he smiled and released Kṛṣṇa. In other words, as Kṛṣṇa, the Supreme Personality of Godhead, binds a living entity in fruitive activities, He binds mother Yaśodā and Nanda Mahārāja in parental affection. This is His pastime.