A phantom center is not using a true center channel and letting the avr send the center channel to the respective L or R channel as needed. Most avr's seem to do this rather well but I still find it less than satisfying the few times I have tried it, especially when listening off-axis.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70