In Odoo relational field Many2many uses special commands in order to save and remove a set of records.  In this post you can find the list of commands for this field and a short explanation for each:

• (0, 0, values)
  Creates a new record from the provided values. The variable values must be of type dictionary.
• (1, id, values)
  Updates the record with specified id with the value in the values.
• (2, id)
  Removes the record with specified id from the database.
• (3, id)
  Removes the record with specified id from the set, but not from the database.
• (4, id)
  Adds an existing record with specified id to the set.
• (5)
  Unlink all records from the set, like using the command “3” on every record explicitly.
• (6, 0, ids)
  Replaces all the existing records in the set by the specified list of records in ids. This one is the same as using command “5” followed by a command “4” for each record in the list.

Commands “1″, “2”, “3”, “5” can not be used in method “create”.

Also, you can find an explanation for every command as a comment in the source code of the models module.