By default as a safety precaution, the switching function is disabled.
You will need to enable it, to activate the video switching function got to the OSD Menu then to sub menu "CAM" and select "SWITCH".
You will need to have your video transmitting and be able to see it.
To access the OSD menu hold down the ENTER button which is located on the front of the top plate just behind the FPV camera.
Please make also sure there is a channel (CH8 on the top plate) from the receiver connected to the cam switch input.