because ingress replication is the older method I think its description is that the ingress linecard makes 6 copies and send them to the interfaces in the oilist for the multicast group.
With egress replication the replication should happen in more scalable manner (this is the advantage) without using 6 times for each packet the crossbar.
In some way the packet takes an internal tag and is taken from the cross bar by all interested modules that look for packets with that tag (and may be for several other ones)
More like a shared memory model.
Note that the requirements are:
sup >= supp720
all modules in chassis must support egress multicast replication.
I had seen a presentation of C7600 architecture but I couldn'f find it
Hope to help