I am rather confused on MVC model-binding and how it interacts with Episerver binding.

The situation is this: I have an episerver page which also has a form that the user will enter data into. I would like to save this user data for a later redisplay of the page.

This works:
--------------------
The episerver model:

   

public class MockPressReleasePage : SitePageData {

    /* whatever episerver properties are needed */

    /* the extra parameters */
        [Ignore]
        public string parm1 { get; set; }
        [Ignore]
        public string parm2 { get; set; }

    }

A model for the extra parameters:
   

public class MockPressReleaseParms {
        public  string parm1 { get; set; }
        public  string parm2 { get; set; }
    }

The controller:

   

public class MockPressReleasePageController : PageController
    {
        public ActionResult Index(MockPressReleasePage currentPage)
        {
            return View("Index", currentPage);
        }

        [HttpPost]
        [AllowAnonymous]
        public ActionResult Submit(MockPressReleasePage currentPage, MockPressReleaseParms parms) {
            var cp = (MockPressReleasePage)currentPage.CreateWritableClone();
            cp.parm1 = parms.parm1;
            cp.parm2 = parms.parm2;
            return View("Index", cp);
        }
    }

The view:

@model MockPressReleasePage

@using (Html.BeginForm("Submit",null,FormMethod.Post)) {
    

        

            @Html.LabelFor(m => m.parm1)
            @Html.TextBoxFor(m => m.parm1)
        
        

            @Html.LabelFor(m => m.parm2)
            @Html.TextBoxFor(m => m.parm2)
        
    
    
}

The key point is that when the Submit action is called, the parms object is bound to the contents of the form textboxes.
-------------------------------------

Of course, this is not how I tried to do it at first. Rather than duplicating all the parameters of MockPressReleaseParms in the episerver model, it would make more sense just to add the whole object (MockPressReleaseParms).
To do this a few changes are needed:

The episerver model becomes:

   

public class MockPressReleasePage : SitePageData {

    /* whatever episerver properties are needed */

    /* the extra parameters */
        [Ignore]
        public MockPressReleaseParms allParms { get; set; }
    }

The controller's Submit action becomes:

       

public ActionResult Submit(MockPressReleasePage currentPage, MockPressReleaseParms parms) {
            var cp = (MockPressReleasePage)currentPage.CreateWritableClone();
            cp.allParms = parms;
            return View("Index", cp);
        }

And the View becomes:

      

 

            @Html.LabelFor(m => m.allParms.parm1)
            @Html.TextBoxFor(m => m.allParms.parm1)
        
        

            @Html.LabelFor(m => m.allParms.parm2)
            @Html.TextBoxFor(m => m.allParms.parm2)
        

----------------------------------
It seems like a trivial change, however now it doesn't work. The key point is that when Submit is called, the binding does not work: in currentPage the 'allParms' property is null, and in 'parms' both 'parm1' and 'parm2' are null.

Some helpful people may ask what I'm trying to do and point out an easier/better way of doing it, and that is fine and welcomed. However what I'm really after is an understanding of how model-binding works so that I can answer this question (and future similar questions) myself.

So why does the model-binding work in the first case and not in the second?