Lockdown is a BYC only expression/word for the 18-thru 21 day of incubation when one is not to dare open or touch the incubator. I don't know where this was ever conjured up but it is a very much talked about subject over to the hatching section.
A hen will naturally stay on the eggs the last few days of the hatch which I'm sure she knows this only by instinct and this is probly triggered by a chirp/cheep or motion inside the egg(s).The last time I checked a hen still can't count too, as I have hens that will brood till doomsday if they are allowed to, so # of days has not a thing to do with it.
Type lockdown in the search box on here you will find plenty of the subject on here for sure.