Engine running, key at IGN 2.
If you then -
Engine off, but only to IGN 1:
All accessories (radio etc) stay on
Engine off, key removed:
All accessories turn off, with exception of HFT which either turns off if no call in progress, or stays on for up to 30 mins if you are still on the phone (which is fun if you then get out of the car
If you then turn the car back on:
When you press the Start button, the 'health warning' screen comes up on the sat nav.
Press 'enter' to agree (that it's not Honda's fault if you crash etc)
Sat nav will then pick up the route from where you are, based on where it was going when you stopped.
Without you needing to do anything