Sadari is said to be the form of Sri Nammalwar, who is also named as “Sadagopan”. As Sri Nammalwar says in his Thiruvaaimozhi, that all our sins will vanish if we strongly attach ourselves to the divine feet, the thiruvadi of Sriman Narayanan. He also explains that this is the universal way of attaining Mukthi (Moksham).
Since, his Thriuvaaimozhi can’t reach all the people in this world but at the same time, they too also has to be shown the way to attain Mukthi, Sri Nammalwar himself is found as “Sri Sadari” in all Sri Vaishnavite temple.
In the Sri Sadari, we can find the thiruvadis (two feet at the top of the Sadari), which are said to be the feet of Sriman Narayanan and this says that Sri Nammalwar is holding the Thiruvadi of Sriman Narayanan on his head.
Because of this only, we should bow down our head to get the Sadari on our head. And this is the regular custom that is followed in every Vaishnavite Temples.
A bhakthan getting himself blessed by the Sadari at Theradi Street, Triplicane at the Utsavam at Sri Parthasarathi Swamy Temple.