The regen should start again the next time the correct exhaust temperature is reached.
The regen STARTS because the ECU decides that a calculation based on number of starts, engine load and average revs means one is required.
By a strategy of controlling the airflow and main injection pulses, the exhaust is heated, then post combustion injections allow diesel to pass down the exhuast to the DPF, further raising its temperature. This continues fro around 10 minutes, when the ECU checks the differential pressure across the DPF until its happy that the filter is adequately clear, then the process stops.
It is interrepted by sitting at idle for more than a few seconds, or switching the engine off.
When its happening you should notice:
1) A slight misfire type vibration (they dont all do this)
2) When moving at speed, if you come off the gas, the turbo boost moves rapidly to zero
3) You get a bassy boom from the exhaust
4) Current MPG plummets
5) Burning smell and red hot exhaust
I try to have about 3000 rpm on during the regen, as more, smaller duration pulses are better and it wont pull any real heavy loads, that could end up with more soot hitting the thing.
If you get stuck in traffic, a light touch on the throttle to keep the car at 1500 rpm will keep it going (watch the turbo boost gauge) if you really must stop and switch off, wait for a minute or two at idle first to allow it to stop.
DONT PARK ON GRASS!