The last three days of incubation the humidity level needs to rise. So on day 18 (chicken) you add water to the incubator to bump up the humidity from 40% to 60-65% and you do not open the incubator until they are done hatching. You stop turning the eggs as well. If the humidity drops it can ruin your hatch. The membrane will "shrink wrap" the chick inside the egg making it impossible for them to move to get out of the egg or breath at all. I hope I am explaining it well enough