Using an HDV camcorder as a video pass through device?

I used to do this a decade ago but I have completely forgotten how to do it!

OK I have the Canon HV20 connected via AV/phonos to the VCR containing the source tape.

The camera is connected via FW to the computer. There is a tape in the camera.

I have tried setting the camera to Play or Record but when I start the VCR playing back nothing appears on the camera screen or the computer's.

I need to get the VCR image into the camera.

I seem to be missing something but I don't know what?