Having ‘Fire in the belly' means being full of energy and enthusiasm for pursuing one's desires, interests and duties in the physical world. When such energy is used for self actualisation it is called 'yoga-agni' or the yogic flame. In this context, Krishna says that some offer all their sense activities and the functions of life force as oblations in the wisdom-kindled yogic flame of inner control of self (4.27).
In day to day life, we offer sense objects like beautiful flowers and tasty food to the paramatma. This verse takes us beyond this and says that yagna (selfless action) is offering sense activities like taste, beauty or smell themselves and not merely sense objects. Senses keep connecting us to the external world through attachment to sense objects and when these senses are sacrificed, what remains is unity with oneness.
Krishna further says that other devotees offer as oblations wealth, self-discipline, and the methods of yoga; while other individuals, self-controlled and keeping strict vows, offer as sacrifices the study of self and the acquirement of scriptural wisdom (4.28).
Krishna mentioned swa-adhyay or study of self as one of the yagna. This process led to the birth of many subjects like psychology, medicine and contemporary self-help classics. Since childhood, we are constantly labelled on factors acquired at birth like nationality, caste or religion and we spend the rest of our lives defending these labels. This labelling involves a lot of suppression or violence at a very young age. Similarly, the labelling happens based on characteristics like intelligent or dumb, hardworking or lazy and the list goes on.
Similarly, we form opinions about ourselves and others based on numerous factors and spend energy defending the same. Swa-adhyay is examining these labels as yagna and shedding them.
Source - Daily World