Crows nest function was the point for me when Jung not only had left the realm of empirical argumentation but logic as well
when you know how brains work, you start to see how much better jungs model fits.
Subliminal perception explains it pretty well. first of all, like jung said about consciousness; "consciousness possesses a level of intensity, so all weak concepts remain unconscious".
"unconscious contains all fantasy images that have yet to encounter suitable conditions to become conscious"
"unconscious is ever present and influences out conscious life"
The deal with subliminal perceptions(including seeing subliminal messages) is that the stimuli is so weak that it doesent reach other areas that would make it conscious, but nevertheless it has an modifying effect on that certain area that processes it, but which doesent communicate it much further. it might communicate it to close areas in cerebral cortex, but not to deeper areas on the brain(which combine all info and thus forms consciousness).
This same effect happens not only with perceptions, but all information exchange in brains, so you might have a thought, which signal is suppressed before it enters consciousness.
Now when it comes to these undeveloped functions, the areas on cerebral cortex which processes the undeveloped function, doesent have direct(or strong direct) link to deeper areas of the brain where consciousness is formed. but they might communicate its message to nearby regions on cerebral cortex.
But because you only come conscious of the last signal sent to areas forming consciousness, you dont become conscious of the undeveloped function, instead the area which is stronger and able to come into consciousness is processing signal from the weaker area.
Example of those would be your visual cortex. you only become conscious of the image that has been already processed, and not about the half processed images.
These undeveloped functions only make a weak signal because, first of all the areas doesent get much imput and therefore they also send a weak signal.
If we take INTPs and Fe, which is basically(or mostly) area that processes social feedback. INTPs with undeveloped Fe doesent register social feedback easily, but when there is a strong social feedback(needs to be stronger than for ESFJ for example) they become very conscious of it, but dont know what to think of this social feedback, doesent understand what it means etc. because the area itself doesent process the info well(low amount of neurons in that area). and because they dont know what to think of it and get confused about it, they try to do further processing with the info and prefer to do the further processing with Ti as its the most developed function of INTP. this means that Fe is processed by Ti and looks like Fi, but Fi is totally different process that doesent even include social feedback.
You should also check this topic about what E and I actually are about in neurological level:
http://www.typologycentral.com/foru...n-introversion-neurological-jungian-view.html
That stuff with brain function in the lenores book has been debunked by dario nardi with his eeg studies on typology(im pretty sure you are aware of that study)