It involves Sacrificing the life of a creature after they are made to drink Somras. The Sacrifice releases the prana and is captured in the somras in its body. Only Tvastr can control Somras at that time as it is exploding with this new pranic energy.
This Somras is then extracted from the Body of sacrificial creature. Whosoever consumes this drink will get the power of the pranic energy. Tvastr tries this technique of Sakra as he is the weakest child of his. To make sure that everybody is benefited from this discovery he chooses 7 Gandharvas and they are known Saptarishis. Brihaspati is one of them.
He secretly teaches this to Trishira too. Later, Trishira discovers a technique to make crystals of this Somras and transfer the power to an inanimate object. He shares this only with Tvastra. And using this technique he creates Vajra.
From Angirasas to vasusEdit
Angirasas were known to control the element of fire. They had the ability to generate and manipulate fire that could be used to create objects from bronze. These angirasas formed a group known as Devas ( the keeper of light), to govern and control the usage of fire. They quickly became powerful, but they were largely made up bythe family of Danu, known as danavas.
After the discovery of prana-boosting properties of soma plant, a new class of gandharvas called Bhrigus led by Aruna became more prominent. With the use of special vessels made from stone, they extracted juice of soma plant, known as somras. Aruna discovered the ability to transfer the prana from nature to another being or object, thus creating new class of powers and powerful objects. But this was heavily dependent on Bhrigus and for a while, they got even more powerful than the Devas.
All this changed when Tvastr, son of one of the Bhrigus, discovered the technique of Somayagya. This gave him the ability to transfer the prana from one body to other animate or inanimate body. He taught this to seven other people, who later came to be known as Vasus. Brihaspati is one of the vasus.
But after Sakras rise as a prominent commander to Syena's army, who were now known as Garudas and were spread across the surrounding hills, the power struggle started between current Devas and another group backed by Garuda army. They tried to align more with vasus, helped largely by the closeness of Sakra and Brihaspati, and this led to discriminatory use of somras and somayagya.
Thus, danavas who were primarily known for their expertise in Fire technology and use of bronze were slowly replaced by a new group of led by Sakra and supported by Garudas.
The technique involved a large amount of intricate rituals carried out by Vasus in presence of the sacrificial offering and the bearer of that sacrifice, also known as Yajmaan. The victim was made to drink the Somras, supplied by Bhrigus. Then the sacrifice was made immediately after and the blood containing its prana was collected in the bowl. The drinker of that blood, usually the Yajman then became the carrier of this prana.
Trishira, son of Tvastr, created a variant of this technique in which the prana could be captured in inanimate objects and that is how Vajra was created.