I have heard I should let my car idle for while before switching off, why is this?

As you may know the turbocharger uses exhaust gases to work and as such it gets extremely hot. If you switch the engine off when the turbo is very hot, say after some hard driving, then there is a chance you could damage the turbo shaft. This is because the shaft in the turbo uses oil from the engine to lubricate it and to keep it cool (note most Evo turbo.s are also water cooled). When you switch off the engine then obviously the oil will not be circulating and if the turbo is still very hot then the oil sitting on the turbo shaft can burn or coke on to the shaft, in very bad cases it can actually block the oil supply or cause heat distortion of the shaft. Allowing the turbo to cool properly before switching off is important for the long life of the turbo.

The easiest way to make sure the turbo is cool is to drive the final 10 kms (or roughly 5 minutes) of your journey with the engine .off boost. (i.e. with the turbo not working hard, just drive gently with no hard acceleration). If you do this then you can quite safely just switch off the engine when you have finished your journey. If you can.t do this then you can just sit stationary with the car on idle for a few minutes to give a chance for the turbo to cool. Driving slowly for the last few kilometers is better for cooling than letting the car sit stationary on idle, it will cool down a lot more quickly when driving due to the air flow through the engine bay. Driving slowly will also give a chance for the brakes to cool down before stopping. It is also important not to rev the engine immediately before switching off or immediately after starting as the turbo will continue to spin due to momentum and there will be little or no oil being supplied to the shaft.