Mary,
Back in the dark ages, I spent a couple of years instructing on a Motorfalke, which had a modified VW engine. It had an electric starter which was utterly reliable. However, to cater for the failure case, we were required by TC to teach "air starts", which involved diving quite steeply, as you described, in order to accelerate to a speed where the prop will turn over - about 90 kts I seem to recall. It was quite disconcerting for pilots who hadn't done it before.