Having hinted that one can attain perfect inaction only by perfect action (Text 18), Krishna now, beggining with this text and continuing through Text 23, guides us on how to perform such perfect, reaction-less action.
Kama sankalpa varjitah - Actions not motivated by selfish desire
How can one act without incurring reaction? How can one live in this world without being tainted by it? The key is selflessness of action.
Jnyanagni dagdha karmanam - Reactions burned by the fire of knowledge
To be free from selfish desire requires knowledge of ones true nature as a part and parcel of God. Such knowledge is like a fire that roasts the seeds of reaction, making them unable to sprout.