Brahman is beyond all action. While performing all action, you are still beyond action. By realizing that i am bramhan, you see the falsity of karma. This is called karma sannasa. Falsity of karma is what is meant by karma sannyasa. How is that accomplished? By gana, by knowledge.
Swami Sarvapriyananda talks on Verse 24 from Chapter 4 of the Bhagavad Gita.