After Create Many to many relationship in crm 2011 it created join entity(Relationship name) in crm data base.
Eg - Account Entity Map With Product Entity using Many to many relationship,then sql create center entity called relationship name(account_product)
When you write the fetch xml to get Account related products the you can use account_product as linked entity.
<fetch version='1.0' mapping='logical' output-format='xml-platform' >
<link-entity name='account_product' from='accountid' to='accountid' >