It happens automatically at various times, not necessarily predictably, but happens least with diesel cars that are regularly driven distances where the heat that puts into a close-coupled DPF brings passive regeneration of the DPF (burns off the soot). If you do a number of short runs then, when you stop, you may be aware of considerable heat under the car. That is from diesel fuel having been pos- injected into the DPF via the engine to start a fire and burn off the soot inside the DPF. You should not switch off the engine while this is taking place. Manufacturers should fit a light warning drivers that active regeneration is taking place and not to switch off, but they don't fit such lights.