C# - How To Use Auto-postback With Checkboxes And Dropdowns

I have a web flow (asp.net) which has a drop down and a check box.

When the check box is ticked, I need to disable some fields in that form.

When a specific value is selected from the check box, I need to disable other fields.

I specify the checkbox like this:

<%=Html.CheckBox("IsResponseUnavailable", Model.IsResponseUnavailable)%>

And the drop down like this:

<%= Html.MyDropDownList(string.Format("Questions[{0}].Answer", i), (IEnumerable<SelectListItem>)ViewData["Periods"], Model.Questions[i].Answer)%>

Where MyDropDownList is an extension of Html.DropDownList

I've heard about auto-postback - but unsure how to use it