Part of that agreement should specify that any idea they have while working on your idea becomes your property. They should also agree to help you in filing patents that might emerge from the work they are doing on your behalf. Corporations will typically ask you to sign their own disclosure agreements. It is helpful to visit manufacturers before a design is complete to try and learn alternative, lower cost, higher quality ways to make your product. This must be done with the utmost respect.
Manufacturers tend to be very linear in their thinking.
Ask them to make a widget that looks like a widget and they'll give you a clear answer based on precisely what you are showing them. They will probably not tell you about the new manufacturing process a competitor is using that could make your widget for less money. Manufacturing sources tend to have little time for conjecture. As you learn more about the best way to make your product you may discover additional things that can be patented. Protecting the way a product is manufactured and assembled, perhaps a notch in a wall that holds a plastic plate, may be the only patent protection you can get.