ASP.NET MVC Session

I read PRO ASP.NET MVC2 from Stephen Sanderson and I still can not figure out something about the session. In the book, he talks about how to design a session-based shopping cart using the Custom model binding for a long session. Everything is working fine, but I can’t understand how it works under the hood. Since this is cool code, I will write a simplified version

Counter

public class Counter
{
    public int counter = 0;

    public void Increment(){
        counter++;
    }
}

Countercontroller

public ActionResult Index(Counter counter)
{
   counter.Increment();
   return View(counter);
}

CounterCustomModelBinder

public class CounterCustomModelBinder: IModelBinder
{
    private const string counterSessionKey = "_counter";

    public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        Counter counter = (Counter)controllerContext.HttpContext.Session[counterSessionKey];
        if (counter == null)
        {
            counter = new Counter();
            controllerContext.HttpContext.Session[counterSessionKey] = counter;
        }
        return counter;
    }
}

Global.asax

...
ModelBinders.Binders.Add(typeof(Counter), new CounterCustomModelBinder());

As you can see, there is an instruction for retrieving the contents of the Counter Counter = (Counter) controllerContext.HttpContext.Session [counterSessionKey] session; But for SAVING, there are no statements in the session. I would expect a subsequent statement somewhere: controllerContext.HttpContext.Session [counterSessionKey] = counter; But this code does not appear anywhere

. - Counter ... , , . , .