When the completion of yajna does not happen in a year (samvatsara) then everything is not stable. Then one has to seek the grace of Vishnu (Vamana) by performing a special rite on the ekadashi day. Yajna means Vishnu (worshipping Vishnu). They perform yajna only for stabilising. They depend on Indra and Agni. Indra and Agni give the abode for Gods (devas). Devas only seek shelter in them and only depend on them.