This feature allows a downstream LSR of a multicast LDP (mLDP) FEC to perform a fast switchover and source the traffic from another upstream LSR while IGP and LDP are converging because of a failure of the upstream LSR which is the primary next-hop of the root LSR for the P2MP FEC. In essence it provides an upstream Fast-Reroute (FRR) node-protection capability for the mLDP FEC packets. It does it at the expense of traffic duplication from two different upstream nodes into the node which performs the fast upstream switchover.
The detailed procedures for this feature are described in draft-pdutta-mpls-mldp-up-redundancy.