A Partial Network (PN) is the mechanism to logically cluster the ECUs. However, the Central or Sub Gateways were the physical clusters in the network. This means a Partial Network is channel agnostics. So, a Gateway ECU has to have a capability to "gate" the activated PN from one channel to the others.
This gating of PN information is called "mirroring" and shall be managed by "External Request Array" (ERA) in two ways in a Gateway ECU - Active or Passive.
With the ACTIVE type gatewaying, the PN information will be assumed just like EIRA that makes the gateway ECU to activate the PN again by itself with the Rx Signal as the source of request rather simply gating the PN information. This will result in the Central Gateway to send the PN activation information to all the channels including the channel it gets the PN originally from. This is termed as "mirroring back" to the same channel. (This actually makes the Central Gateway as the last ECU to release the PN)

Consider the network above, if a PN is being activated by an ECU in Body-CAN-1, then the CGM will activate the PN in itself and sends the activated PN information over BusNM to all the channels connected to it, including the Body-CAN-1 channel.
P.S.: I never came across the PASSIVE gateway, but from a brief reading, it seems, it is something that simply gating rather activating the PN.