physiological reasoning behind lockdown?

Good questions, and pretty much the same concerns that I had with my first hatch (which is completing today! woohoo!)

From what I've read, the chick starts trying to position itself for internal pipping (breaking into the aircell and starting to breathe) on day 18. Turning the egg may actually hinder it's progress to do so, since it literally has to turn around in the egg to face towards the aircell. Just thinking about having to turn around in a tight space while it rotates makes me seasick.

Also, it's crucial at this point that humidity be maintained, and depending on your location, opening the incubator to turn will lose you precious humidity and temperature.

And lastly, until an external pip forms, it's a guessing game to know where the chick will be coming out. The last thing you want happening is a chick working for hours to pip, and then you unknowingly rotate the egg so that the potential pip is pressed against the floor. Hatching takes massive amounts of energy, and is not something they can just do-over if their first pip puts them against an umovable surface with no access to oxygen.

Hope that helps to explain it! And I'll send good hope vibes for the success of your next hatch!

 

Maybe the links in my signature would answer your question.

 

In those last 3 days, the humidity is raised, because if the humidity is at 40%, and you see a pip, you are probably already too late to bring the humidity up. I think the 3 day thing is to let the incubator humidity remain at a constant, and stabilize to a certain percent. As for the turning, I am guessing that, because in most of the old cabinet incubators, and the big commercial incubators, the eggs are tilted. And they are put in a hatcher on their side to hatch. In those last 3 days the chick is repositioning itself to be able to hatch. I can take a guess, and say that the turning wouldn't be beneficial for the chick if it was trying to reposition itself.