The environment under a hen in a coop and subjected to weather patterns is vastly different than the air and temp inside the house and inside the incubator. The blast of air the comes through the incubator is different than when a hen rises off the nest. In Spring/Summer.. she may be hopping off the nest in 92 degree humid as heck weather. In the house when you open the lid, it'll be dry a/c at about 70 or so. Cold enough and fast enough air to shrink wrap a pipped chick in the incubator with the sudden change.
Incubation do's and don'ts are subject to environment and conditions. "Lockdown" is fool proof regardless... the eggs don't need turned, it can make it more difficult on the chicks to shift around. Humidity keeps the membrane moist to avoid stuck chicks that can't shift in the egg well when it dries out. The heating element of an incubator is very drying.. not so under a broody. Leaving the lid shut and not opening it (thus the term lock down) ensures that you won't send a cold, dry blast of room temperature air in that could shrink wrap the membrane around the chick.
Different methods have different guidelines... with a broody you don't have to worry much, just if she's a good mother or not and that she's being diligent with setting. Once you go artificial, you can't treat the incubator like a broody hen.. it's a different environment for the eggs.
At least that's my take on it. Thinking back on it, I don't remember my broodies getting up when hatching started. Maybe hens have a lockdown period too.
