[Advaita-l] शुक्लयजुर्वेदीय सन्ध्याप्रयोगः

[Animesh]

Srī mātre namah ,

Namaste Chandrasekhara ji,

It is quit interesting to observe what your tradition have preserved.
I think this may be a kula paramapara ( family tradtion ) to take tear a portion of angavastra thread and place underneath so as to sit on it.This is not mentioned in any of printed text as you mentioned, I think it is unique pratice to your family tradition and you should keep it up .( I will encourage you to keep it as it is lokAchara of your cast ( jAti ) and you should have trust on sayings of elders ).There are many rules which are unique and peculiar to some families and they should be followed even if they are not mentioned in texts .
Even in  sandhyA  of our family we recite  suryachh... Mantra. ( we are basically atharvavedins and in our procedure it is not mentioned. But in our family tradition this  is done, even if it is not mentioned in sandhayA parishIstha )
This is unique practice of our tradition.On Sanskrit documents website I have seen this pratice in rigveda sandhya vandan of mAdhwa followers .But the mantras are different .

Regards 
Animesh