On Thu, 11 Nov 2004, Bob Hoover wrote:

Now wire the solenoid valve into the ignition circuit. (Circuit ON,
valve OPEN)

If the valve doesn't close until you turn off the mag switch, then you
can't shut down by pulling the mixture, or you lose the oil pressure. So
there's still fuel in the cylinders, which could ignite if a P-lead is
broken and someone turns the prop. There's not a huge chance of that
happening, but it would still be safer to have the valve on a manual
switch that you turn off white the engine's still turning.

-Dan