Forced regeration, ok, natural regeneration occurs when certain circumstances occur during driving, cruising with a hot engine etc etc. If those circumstances do not happen (short journeys, city driving etc etc) then the system cannot clean that dpf, so after monitoring the pressure in the dpf the warning light comes on and it goes to your dealer.
The dealer checks through everything and then carries out a forced regeneration using examiner. They stick the car outside or up on a ramp (you need as much cold air as possible around the dpf) and if inside fit a remote exhaust extractor (if you fit it directly to the exhaust it would melt). Secure an area around the car so no one gets too close, and start the regeration, the engine has to be hot, no pedals pressed and a load on the engine (lights etc). Examiner revs the engine to 2000 rpm and monitors the parameters to make sure its safe to do a regen. After this check, the engine is then revved to 3000 rpm and regen starts, it takes approx 15 minutes and sparks and soot are ejected from the exhaust. The injection period is extended and post injection occurs which super heats the exhaust and dpf, raising the temperature to between 600 and 800 degrees c, burning off the particulates that are trapped in the filter. After regen has occurred the vehicle is driven hard to flush out as much soot as possible, typically the pressure difference between before and after regen in the dpf goes from 150 mbars to 10 mbars at idle. This pressure difference increases as the revs are raised.