That eBay one is the same as the one I linked to on the Ascott site. I had one when I lived in Exmoor with now XH and I bought another as soon as I had chickens again.
I mount it, not on a wall, but on a long (4') pole, like a fencing stake but with square sides. I can then take it to wherever I may need to use it, lean it up something stable - like a wall or gate - and use it there. I also find it easier to do on my own this way, but that's maybe just me, I am not dextrous.
It does work reliably, yes, it seperates two vertebrae thereby breaking the neck (severing the spinal cord) and not crushing the spine. Hold the bird firmly round its body to contain the flapping; when it's still, hang it up by the feet and let the blood pool into the wound in the neck. You can pluck it while it's upside down, or I just pluck it straight away and then hang it upside down.
I have never felt the need for a backup when I use this - but I have found that I need to take a bit of care exposing the part of the neck where I am going to close the jaws when I am doing a large cockeral with plenty of neck feathers. So if I had a very large bird, larger than I've done before, then I would have a backup handy.
I will be very interested to hear what you learn on your course, plums.