Ok. I understand the confusion. It is dunking a sick hen in water that can shock and kill them. It has nothing to do with broodiness. The issue for me is that many times they are sick and hide it from us. There is no way I'm going to risk killing one of my hens in that way.
Also, broody hens tend to have deprived themselves. Who knows if they are more susceptible or not. I can't imagine it is really safe to dunk them.
My keeping circumstances may well produce a different outcome. Very approximately one third by weight of a hens food consumption goes to produce an egg.
All the broody hens here start to increase their daily food intake before they sit. I have tried weighing a couple but the increase in feed consumption doesn't produce a definite increase in body weight over the four days I noticed an increase in food consumption. It's a matter of grams and as such not a reliable indicator.
I've noticed a change in feeding both in quantitiy and selection up to four days before a hen has sat on her clutch. It may start earlier, particularly in feed selection.
The hen knows she is going to sit on her clutch some time before actually sitting. How far in advance is an interesting topic.
She also knows that once she has sat the opportunity for feeding is reduced. Those first few days, not only is her feeding opportunity reduced, but she may also lay eggs for up to three more days. It makes sense to have a store of nutrients above her normal daily requirements.
She also knows that once the chicks have started to hatch she may be confined to the nest for 48 hours of more and once hatching is complete, the vast majority of any food she finds will get fed to the chicks. It makes sense therefor for a hen to try to maintain her feed intake when sitting. In theory at least, if she is not having to divert nutrients to egg making then she can consume one third less than usual and still maintain a healthy body weight.
I have not detected any noticeable reduction in a sitting hens body weight here when sitting on eggs. Granted I make sure that food is available but even in the wild, hens choose their reproduction time partly by season. The spring for example is likely to provide better foraging opportunities than winter.
I have a rough idea of the quantity of feed consumed by a hen on a daily basis.
A not particularly pleasant experiment was to weigh the notorious broody poop. It seems that what comes out in one or two goes is very similar to what went in by weight and that is very similar to what she would consume daily when not broody, but reduced by a little under a third.
It would seem therefore that a hen given a adequate forage/feed is able to maintain her bodyweight and the notion that sitting is hard on a hen and they necessarily lose weight and condition is false.
This makes sense for species survival and can be seen in humans and other creatures.
Pregnant women often eat more and put on weight while they are pregnant. This is a natural response to preparing for feeding the child while having reduced food for herself. Many creatures do this when preparing to raise young.