Dry powder extinguishers used to be sodium bicarbonate and maybe some still are but a different powder (a phospate ??) is now used to make them suitable for a wider range of fire types.( AB&C rather than just B&C ???)thank DeepJoy for confirming what I had read somewhere but couldn't put my finger on, that the powder is Sodium Bicarbonate.
IIRC it melts and encases the "fuel" when that is solid and so seals it off and helps prevent re-ignition. Bicarb was not good at preventing re-ignition of solid fuels eg wood.