There is an interesting legend as to why the first Pavamaana Sthuthi of Praathassavanam is done outside, and hence called "Bahish-Pavamaana-Sthuthi".
Unlike other Devans (gods), Aswineedevans were not permitted to do the Soma part of Yaagam, since they had been ostracised or ex-communicated (Bhrashtu) on account of Bhaishajam, but consequently, the aged seer, Chyavana Maharshi, is said to have regained youth and eyesight. To show gratitude, he promised to offer Soma in their Yaagams.
King Saryaathi, son of Manu and son-in-law of Chyavanan, decided to perform a Somayaagam, and appointed his father-in-law as the chief priest. Arrangements were made for Aswineedevans to get Soma, though not with others. This was in spite of protests from the other gods, including Indran. It was, however, decided that they should do Praayaschitham, since unconditional re-entry after such a long period was considered unethical and against Dharma Saasthram tenets, and the Praayaschitham was: listen to Pavamaana-Sthuthi ! (Pavamaanam means cleansing).
The Riks used for Pavamaana-Sthuthi are all taken from the Pavamaana section of Rigvedam. Now, since Aswineedevans were then out-castes, they were banned from entering the Vedi. So the location of Sthuthi was moved to the extreme end of the Vedi to enable them to hear while sitting outside. The practice of conducting the Sthuthi in this manner continues to this day !
Aarti / Stuti