The document discusses using React mixins to allow for dynamic form fields that can register and deregister with a parent form component.
The author is having issues with a form where fields can be dynamically added and removed, but the form needs to know if it can be submitted. React mixins are proposed as a solution to allow components to share common functionality while remaining separate.
The solution involves a form component that registers input fields in an object, and can deregister them. It also checks validity when fields register/deregister. Input components can register and deregister themselves from the form.