value_before_type_cast

I have created a plugin that returns a Flash text editor and a hidden
input field. I'm using an instance of the InstanceTag class to create
a custom form helper for the hidden field. I need to be able to
manipulate the value that is put into the hidden filed but I don't
understand how the value_before_type_cast method in this class returns
a value. Can anyone shed some light on this for me?