On a VW diesel, when regeneration occurs the idle is boosted from 800 to 1600 rpm and the engine sprays a extra shot of fuel into the exhaust when the exhaust valve is open. This lights-off the particulate filter and the filter burns at 1,500 F degrees for a few minutes. Then all returns to normal. If at speed, you never know it is happening. The only prohibition is stay of dead grass fields.