Namaste,
While I can't explain what *exactly* happens in yasna, as I know no practicing zoroastrians where I live and never have attended any rituals, from what I could gather as information yasna is basically any lithurgy to contemplate on & celebrate Ahura Mazda and his goodness / creations. (Including offering, prayers, worship, confession, etc.)
From what I understand about Hindu yagna, it could be similar?
*hopes someone knowledgeable of both faiths replies soon. >.>*