The PIM multicast router presence detection feature scans the network traffic for incoming PIM hellos. This feature is enabled when multicast routing or snooping is enabled.
When a PIM hello is detected, that port is marked for the presence of a multicast router and the information is saved. This action prevents unnecessary flooding if the PIM designated router (DR) goes offline, because IGMP reports are forwarded to the multicast routers and not only the snooping-enabled router.