Gives you a [name]_attributes= setter method to create nested objects via params.
Example
contact.rb
builds_on :emails, :field_to_be_skipped_if_blank => "address"
contacts/_form.html.erb
<%= text_field_tag "contact[email_attributes][][address]" %>
Options
name- Name of association in singular form. I.E. address, email, etc.field_to_be_skipped_if_blank- If field is left blank, it will remove it from the attributes before inserting.destroy_associations- Will destroy all existing associated objects before insert.