The answer to your question "what keeps the engine from starting?" is NOTHING, be careful.

I personally would never rotate the engine with the starter when adjusting the valves. I use a 27MM socket on the crankshaft bolt. On a 240D it's much easier than on a five cylinder, but I do it this way on a five cylinder as well. By turning with a ratchet, you turn it slow and there's really no way that the engine will start. Bumping the starter can indeed start the engine.

Even though my right hand looks like it went through a meat grinder after adjusting valves on a five cylinder, I personally would not do it any other way, too dangerous for my wimpy risk taking abilities.

What kills these engines is vacuum applied to a diaphragm at the injection pump. IF the vacuum system is tight and reservoir in good shape, it SHOULD not start with the ignition switch in the off position, but this seems like a risk to me. I think I would only assume this to be safe if I took off the spider, that is the injection lines that connect the injection pump fittings to the individual injectors. If you do this you will have to prime and restart the engine when done. Thank you no, I'll use the 27MM socket.

Doug, I'm not trying to be my same old opinionated, anally retentive self here, rather, I'm trying to ensure that you understand the situation for your own safety.

Good luck,