How To Make A Custom Strongly Typed Html Helper Method

Now I found out how to create custom html helpers

using System;
namespace MvcApplication.Helpers {
public class InputlHelper {
public static string Input(this HtmlHelper helper, string name, string text) {
return String.Format("<input name='{0}'>{1}</input>", name, text);
}
}
}

Now how to turn it into a strongly typed helper method InputFor Like it is in the framework?I don't need the Html.TextBoxFor method, I know it exists. I am just curious in how to implement this behavior myself and used this as a simple example.PS. I was looking in the mvc source code but couldn't find a trace of this mysterious TextBoxFor. I only found TextBox. Am I looking at the wrong code?