You wet the bread crumbs down so they can be moulded into a dough - that dough will survive being cast out and getting to the bottom. Then it will dissolve, get washed out by current out or be pecked out by fish
but yeah, you want the burley to fall out eventually - you just have to get the right dampness of the mix - so it falls out on the bottom and not while being cast.
I had the mix in this vid about perfect before I added the corn - then it got a bit rubbery - so you have to watch how much moisture goes in or it will clump and not be released