Chapter IV Sloka 30

Apre inytahara> a[aNa[e;u juit,
sveR=Pyete y}ivdae y}]iptkLm;a>

apare niyathr 
prnpreu juhvati |
sarve'pyete yajavido 
yajakapitakalma ( 4|30)||

Others who regulate their diet offer life-breaths in life-breaths. All these are knowers of sacrifice, whose sins are destroyed by sacrifice.

Commentary:

Niyataharah means persons of regulated or limited food. They take moderate food. By rigid dieting they control the passions and appetites by weakening the functions of the organs of action.
Yogis pour the life-breaths as sacrifice in the controlled life-breath. The former becomes merged in the latter.
Chinmayananda:

All these "knowers of yagna", meaning those who know "the art of living these techniques", can fully come to profit themselves when they practise them in a spirit of self-dedication and selfless enthusiasm. These methods, of themselves, will not guide us or lead us to the Supreme. What is assured is that those who practise atleast one or a few/all of them for a sufficiently long period can become "purified of sins." Sinful vasanas make the ego act like an animal and go along the wrong path. The above mentioned practices will wipe clean the existing wrong-vasanas and cut out in their place, new channels of thoughts more constructive for spiritual efforts to succeed. Arjuna has been instructed that all paths, however noble  and great, are but means and not an end in themselves.