The following figure shows the red link between CE-3 and CE-4 could be a low speed OC-3/STM-1 link but because it establishes a intra-area route connection between the CE-3 and CE-4 the potentially high-speed PE-1 to PE-2 connection will not be utilized. Even with a super backbone configuration it is regarded as a inter-area connection.
The establishment of the (green) sham-link is also constructed as an intra-area link between PE routers, a normal OSPF adjacency is formed and the link-state database is exchanged across the MPLS-VPRN. As a result, the desired intra-area connectivity is created, at this time the cost of the green and red links can be managed such that the red link becomes a standby link only in case the VPN fails.
A sham link is only required if a back door link (shown as the red link in the preceding figure) is present; otherwise, configuring an OSPF super backbone will probably suffice.