This was tested on a Canon ZR40 and a Mac running Yosemite, but it worked on a Mac with 10.6.8 (Snow Leopard) as well. I am told that the same approach works with other similar models of camcorder.
1. Connect the firewire cable from the camcorder to the computer.
2. Open iMovie, make sure it sees the camera: "ZR40 (DV)" and import settings are set to Manual.
3. Connect the Camcorder to the VCR via the Stereo Video Cable -- Yellow, Red and White to VCR "OUT", 1/8" end to camcorder AV port.
4. Turn on the VCR and load your recorded cassette, and put the VCR in playback pause mode.
5. Turn the Camcorder's POWER switch to PLAY (VCR) mode.
6. Load a blank cassette in the Camcorder.
7. Open Camcorder's Menu, go to VCR setup, set it to ??? FIXME A/V passthru mode
8. Press the REC PAUSE button on the camcorder's remote.
9. Start the iMovie import event.
10. Press the camcorder's FOCUS/Play/Pause button start recording. YES, YOU HAVE TO RECORD TO CAMCORDER'S TAPE in order to pass it through to the computer. Otherwise the camcorder goes to sleep after a minute of being idle or something.
11. Start playback on the VCR.
12. Stop playback on the VCR once the recording is done in the tape.
You can monitor the picture in the camcorder’s viewfinder or LCD, or in iMovie's import window.
— MB August 2016