If they are switched off when red-hot the oil trapped in their bearings instantly carbonises and as the turbo spools down the bearings wear. If you continue to idle the engine before switching off, oil is pumped through the turbo bearings, cooling and lubricating them. This is usually only necessary when stopping for fuel at a motorway service station, after a long ascent, or after towing. Normally the end of a journey at relatively low rpm, for example through a town, does the same job.