Resetting formcheck for mootools on checkbox change

I have a form that uses [formcheck][1] (a MooTools class that allows you to perform checks on validation) everything works fine, I have also built a part of the form that can be hidden, (by using mootools' 'slide' class) on a checkbox selection. The problem is that the fields that are hidden are linked into the formcheck class, and are registered when the page loads via: