Неворошенный жар под пеплом лежит

Some active and live energy is often hidden under a thick veil of inertness or restraint See И в пепле искра бывает (И)

Cf: The fire in the flint shows not till it's struck (Br.). The fire in the flint hows not until it is struck (Am.). Hidden fires are always the hottest (Am.). In the coldest flint there is hot fire (Br.). Strike a flint and you get fire (Am.)