At the beginning of creation, all angels(devatas) were suffering from hunger. They sought help from Prajapati(Brahma). Prajapati said, 'My job is to create. Lord Vishnu is the master of sustaining the creation. So I suggest you approach him in this regard'.
Devatas approached Lord Vishnu as per Prajapati's advise. Vishnu said, 'O! Devatas, here onwards, the havis (Oblation) offered by Brahmins and Kshatriyas while performing yagna will be your food. This havis is added to the fire inside the yagna kunda. It will satisfy all your hunger and gives you the required strength. There is a shakti called 'Swaha' who can help to carry these havis from the fire to you. Pray Divine mother(master of all Shaktis). She will send 'Swaha' for your help'.
Devata's prayed divine mother with devotion. Divine mother was pleased by their prayer and sent 'swaha'. Prajapati said this to Swaha,' O! Mother, you please serve as the burning power of the fire god and help us in transferring the havis offered in the yagna to all the devatas'. Then Swaha said,' I belong to Lord Vishnu. I cannot get bonded with fire god. I am sorry'. Saying this, she went away and started praying for Lord Vishnu. After some time, Lord Vishnu appeared in front of Swaha and said, 'My dear Shakti, I understand your wish. In your next birth, you will be born as 'Nagnajiti', daughter of 'Nagnajit'. I will come as 'Sri Krishna'. Then you will marry me. For now please join fire god and become his burning power'. From then onwards, swaha joined fire god. They had 3 childeren. They are: 1. Aahavaneeyagni 2.Gaarha patyam, 3.Dakshinaagni.
Like this, by grace of shakti, 'Swaha', the hunger of all the devatas is satisfied. So whenever a havis is added into the fire of the yagna, we call the name of the devata and swaha together. Like..
