I would just let 'birth' take it's natural course with no interventions. Intervening usually will cause more problems than it will solve. If the ducklings are strong enough to break out, they will. Usually when the eggs are rolling around they are trying to get into a better position to break out. Assisting with a hatch is not advisable, if you try to 'help' you can end up causing more harm. If the chick is weak or deformed, it won't live, which in the long run is better for it. As long as the water level in the incubator is high enough, it should have enough humidity I guess.