This switch controls whether multicast traffic is flooded to all ports when no IGMP Snooping member exists for its destination group. Enabling it suppresses that flood-when-unknown fallback, so unregistered multicast streams are dropped instead of blasted across the VLAN, but per the description it only takes effect once IGMP Snooping itself is enabled. An admin on a bandwidth-constrained switch would enable this to stop an unsubscribed multicast source, such as a misconfigured server, from flooding traffic to every port.