Bidirectional PIM is designed for scenarios where there are many sources and receivers that need to communicate with each other, such as in videoconferencing. Unlike traditional PIM sparse mode, which creates both (,G) and (S,G) entries, Bidirectional PIM only allows the (*,G) entry for the shared tree and does not build an (S,G) entry. This approach reduces the number of mroute state entries, conserving resources.
Traffic can flow up and down the shared tree in Bidirectional PIM, and there is no RPF check; instead, a Designated Forwarder (DF) is used to prevent loops. References := Multicast Bidirectional PIM - NetworkLessons.com