Ellsworth Wente

I think Porsche wants you to cool it down a bit at idle because turning it off immediately removes the cooling air flow. So after an extended hard drive it will actually get hotter for a minute as the heat inside the engine conducts out and the heat sink fins can't remove the heat as well without air moving over them. So running it down a bit will lower the overall temp before the shutdown. For example, if you have a turbo charger glowing cherry red (like the one on my dad's old turbo t-bird when I was 16 - don't ask), then removed the air flow that's keeping it cherry red rather than white hot, it'd get real hot for a little while.... (of course this is speculation on my part... just have a physics backgound)
ell