Ignition with the ignition off is caused by latent heat of the head/block plus the heat of compression,(think how hot your air compressor head gets).. That's why it happens on engines that have been run hard, like towing on a hot day. The head temp is probably around 250 and coolant is 240 or so. Slowing the idle reduces the amount of air/fuel and reduces the heat of compression. If you have ever worked with diesels you know how hard they are to start with low batteries, same principal, slow revs lower heat of compression.
