I'm not sure how many locos this feature has appeared on aside from the VL Challengers. On that model, the feature runs continuously from an auxiliary smoke unit. It can be turned off via a switch on top of the boiler or via CAB controls. The latest manual contains these instructions:
There is a sound of the dynamo spooling up when the engine starts but that is part of the background loco sounds and not independent of them.
Certain of the GS series VL engines will have this feature in substitution for whistle smoke because (as the catalog and related video explain), where the whistle and stack are close together the stack smoke effectively masks the whistle output. Also, as I read the catalog description these models will have smoke from all three dynamos lined up on the boiler.
Personally, I would prefer whistle smoke where the whistle is not right next to the stack but I understand the reasons given for substituting the dynamo effect.