The Procedure of Sr&ddha1 - Verse 10-11

What Sraddhas are to be offered (to Pitrs) ? How do these Sraddhas reach Pitrs—the Sraddhas that are offered uttering the names of father, father’s father and greatgrandfatliter against the three rice-balk (Pindas) ? How are they (Pitrs) , if stationed in hell, competent to grant benefits ?